Here are some of the teachings of the Fathers and Doctors of the Church concerning this terrible and sinful disposition, to which I also have found myself blindly enslaved (may it be ever behind me by the grace of God and the help of His holy and sinless Ever Virgin Mother!):
St. Thomas Aquinas, Summa Theologica > Second Part of the Second Part > Question 20, Despair:
Objection 1. It would seem that despair does not arise from sloth...
On the contrary, Gregory (Moral. xxxi, 45) reckons despair among the effects of sloth. I answer that, As stated above (17, 1; I-II, 40, 1), the object of hope is a good, difficult but possible to obtain by oneself or by another. Consequently the hope of obtaining happiness may be lacking in a person [...] the fact that a man deems an arduous good impossible to obtain, either by himself or by another, is due to his being over downcast, because when this state of mind dominates his affections, it seems to him that he will never be able to rise to any good. And since sloth is a sadness that casts down the spirit, in this way despair is born of sloth."
Catholic Encyclopedia, Sloth:
"One of the seven capital sins. In general it means disinclination to labour or exertion. As a capital or deadly vice St. Thomas (II-II:35) calls it sadness in the face of some spiritual good which one has to achieve (Tristitia de bono spirituali).
"Father Rickaby aptly translates its Latin equivalent acedia (Gr. akedia) by saying that it means the don't-care feeling. A man apprehends the practice of virtue to be beset with difficulties and chafes under the restraints imposed by the service of God. The narrow way stretches wearily before him and his soul grows sluggish and torpid at the thought of the painful life journey. The idea of right living inspires not joy but disgust, because of its laboriousness.
"This is the notion commonly obtaining, and in this sense sloth is not a specific vice according to the teaching of St. Thomas, but rather a circumstance of all vices. Ordinarily it will not have the malice of mortal sin unless, of course, we conceive it to be so utter that because of it one is willing to bid defiance to some serious obligation. St. Thomas completes his definition of sloth by saying that it is torpor in the presence of spiritual good which is Divine good.
"In other words, a man is then formally distressed at the prospect of what he must do for God to bring about or keep intact his friendship with God. In this sense sloth is directly opposed to charity. It is then a mortal sin unless the act be lacking in entire advertence or full consent of the will. The trouble attached to maintenance of the inhabiting of God by charity arouses tedium in such a person. He violates, therefore, expressly the first and the greatest of the commandments: "Thou shalt love the Lord thy God with thy whole heart, and with thy whole soul, and with thy whole mind, and with thy whole strength." (Mark 12:30)."
St. Thomas Aquinas, Summa Theologica > Second Part of the Second Part > Question 35, Article 3. Whether sloth is a mortal sin?
Objection 1. It would seem that sloth is not a mortal sin. For every mortal sin is contrary to a precept of the Divine Law. But sloth seems contrary to no precept, as one may see by going through the precepts of the Decalogue. Therefore sloth is not a mortal sin.
Objection 2. Further, in the same genus, a sin of deed is no less grievous than a sin of thought. Now it is not a mortal sin to refrain in deed from some spiritual good which leads to God, else it would be a mortal sin not to observe the counsels. Therefore it is not a mortal sin to refrain in thought from such like spiritual works. Therefore sloth is not a mortal sin.
Objection 3. Further, no mortal sin is to be found in a perfect man. But sloth is to be found in a perfect man: for Cassian says (De Instit. Caenob. x, l) that "sloth is well known to the solitary, and is a most vexatious and persistent foe to the hermit." Therefore sloth is not always a mortal sin.
On the contrary, It is written (2 Corinthians 7:20): "The sorrow of the world worketh death." But such is sloth; for it is not sorrow "according to God," which is contrasted with sorrow of the world. Therefore it is a mortal sin.
I answer that, As stated above (I-II, 88, 1,2), mortal sin is so called because it destroys the spiritual life which is the effect of charity, whereby God dwells in us. Wherefore any sin which by its very nature is contrary to charity is a mortal sin by reason of its genus. And such is sloth, because the proper effect of charity is joy in God, as stated above (Question 28, Article 1), while sloth is sorrow about spiritual good in as much as it is a Divine good. Therefore sloth is a mortal sin in respect of its genus. But it must be observed with regard to all sins that are mortal in respect of their genus, that they are not mortal, save when they attain to their perfection. Because the consummation of sin is in the consent of reason: for we are speaking now of human sins consisting in human acts, the principle of which is the reason. Wherefore if the sin be a mere beginning of sin in the sensuality alone, without attaining to the consent of reason, it is a venial sin on account of the imperfection of the act. Thus in the genus of adultery, the concupiscence that goes no further than the sensuality is a venial sin, whereas if it reach to the consent of reason, it is a mortal sin. So too, the movement of sloth is sometimes in the sensuality alone, by reason of the opposition of the flesh to the spirit, and then it is a venial sin; whereas sometimes it reaches to the reason, which consents in the dislike, horror and detestation of the Divine good, on account of the flesh utterly prevailing over the spirit. On this case it is evident that sloth is a mortal sin.
Reply to Objection 1. Sloth is opposed to the precept about hallowing the Sabbath day. For this precept, in so far as it is a moral precept, implicitly commands the mind to rest in God: and sorrow of the mind about the Divine good is contrary thereto.
Reply to Objection 2. Sloth is not an aversion of the mind from any spiritual good, but from the Divine good, to which the mind is obliged to adhere. Wherefore if a man is sorry because someone forces him to do acts of virtue that he is not bound to do, this is not a sin of sloth; but when he is sorry to have to do something for God's sake.
Reply to Objection 3. Imperfect movements of sloth are to be found in holy men, but they do not reach to the consent of reason.
St. John Chrysostom, Third Homily on 2nd Thessalonians:
"For as in the case of husbandmen, the seeds are indeed cast into the earth once for all, yet do not constantly remain, but require much preparation withal, and if they do not break up the earth, and cover over the seeds sown, they sow for the birds that gather grain; so we also, unless by constant remembrance we cover over what has been sown, have but cast it all into the air. For both the devil carries it away, and our sloth destroys it, and the sun dries it up, and the rain washes it away, and the thorns choke it: so that it is not sufficient after once sowing it to depart, but there is need of much assiduity, driving off the birds, rooting up the thorns, filling up the stony ground with much earth, checking, and fencing off, and taking away everything injurious. But in the case of the earth all depends upon the husbandman, for it is a lifeless subject, and prepared only to be passive. But in the spiritual soil it is quite otherwise. All is not the teachers' part, but half at least, if not more, that of the disciples. It is our part indeed to cast the seed, but yours to do the things spoken for your recollection, by your works to show the fruits, to pull up the thorns by the roots."
St. John Cassian, Insitutes, Book 3, Chapter 5 (How they ought not to go back to bed again after the Mattin [morning] prayers.):
"But some in this province, not knowing the reason why this office was appointed and introduced, go back again to bed after their Mattin prayers are finished, and in spite of it fall into that very habit to check which our Elders instituted this service.
"For they are eager to finish it at that hour, that an opportunity maybe given, to those who are inclined to be indifferent and not careful enough, to go back to bed again, which most certainly ought not to be done (as we showed more fully in the previous book when describing the service of the Egyptians), for fear lest the force of our natural passions should be aroused and stain that purity of ours which was gained by humble confession and prayers before the dawn, or some illusion of the enemy pollute us, or even the repose of a pure and natural sleep interfere with the fervour of our spirit and make us lazy and slothful throughout the whole day, as we are chilled by the sluggishness caused by sleep.
"And to avoid this the Egyptians, and especially as they are in the habit of rising at fixed times even before the cock-crow, when the canonical office has been celebrated, afterwards prolong their vigils even to daylight, that the morning light when it comes on them may find them established in fervour of spirit, and keep them still more careful and fervent all through the day, as it has found them prepared for the conflict and strengthened against their daily struggle with the devil by the practice of nocturnal vigils and spiritual meditation."
St. John Chrysostom, Instruction to Catechumens:
"For strengthened not by our own power alone, but by the influence which comes from God, we are summoned to the conflict. Let as many therefore as have received what has been spoken, and have fulfilled it by their works, remain reaching forth to the things which are before. But let as many as have not yet arrived at this good achievement, arrive at it straightway, that they may dispel the condemnation which arises out of their sloth by their diligence for the future. For it is possible, it is indeed possible for him who has been very slothful, by using diligence for the future to recover the whole loss of the time that is past. Wherefore, He says, Today if you will hear his voice, harden not your hearts, as in the day of provocation.
"And this, He says, exhorting and counseling us; that we should never despair, but so long as we are here, should have good hopes, and should lay hold on what is before us, and hasten towards the prize of our high calling of God.
"This then let us do, and let us inquire into the names of this great gift. For as ignorance of the greatness of this dignity makes those who are honored with it more slothful, so when it is known it renders them thankful, and makes them more earnest; and anyhow it would be disgraceful and ridiculous that they who enjoy such glory and honors from God, should not even know what the names of it are intended to show forth.
"And why do I speak about this gift, for if you will consider the common name of our race, you will receive the greatest instruction and incentive to virtue. For this name Man, we do not define according as they who are without define it, but as the Divine Scripture has bidden us. For a man is not merely whosoever has hands and feet of a man, nor whosoever is rational only, but whosoever practices piety and virtue with boldness. Hear, at least, what he says concerning Job. For in saying that there was a man in the land of Ausis, he does not describe him in those terms in which they who are without describe him, nor does he say this because he had two feet and broad nails, but he added the evidences of his piety and said, every just, true, fearing God, eschewing evil deed, (Job 1:1) showing that this is a man; even as therefore another says, Fear God, and keep his commandments, because this is the whole man. (Ecclesiastes 12:13)
"But if the name man affords such a great incentive to virtue, much rather the term faithful. For you are called faithful on this account, because you have faith in God, and yourself art entrusted from Him with righteousness, sanctification, cleansing of soul, adoption, the kingdom of heaven. He entrusted you with these, and handed them over to you.
"Thou in turn hast entrusted, and handed over other things to him, almsgiving, prayers, self-control and every other virtue. And why do I say almsgiving? If you give him even a cup of cold water, you shall not indeed lose this, but even this he keeps with care against that day, and will restore it with overflowing abundance. For this truly is wonderful, that he does not keep only that which has been entrusted to him, but in recompensing it increases it."