Objection 1. It seems that constancy does not pertain to perseverance. For constancy pertains to patience, as stated above (Question 137, Article 5): and patience differs from perseverance. Therefore constancy does not pertain to perseverance.
Objection 2. Further, "virtue is about the difficult and the good." Now it does not seem difficult to be constant in little works, but only in great deeds, which pertain to magnificence. Therefore constancy pertains to magnificence rather than to perseverance.
Objection 3. Further, if constancy pertained to perseverance, it would seem nowise to differ from it, since both denote a kind of unchangeableness. Yet they differ: for Macrobius (In Somn. Scip. i) condivides constancy with firmness by which he indicates perseverance, as stated above (128, 6). Therefore constancy does not pertain to perseverance.
On the contrary, One is said to be constant because one stands to a thing. Now it belongs to perseverance to stand to certain things, as appears from the definition given by Andronicus. Therefore constancy belongs to perseverance.
I answer that, Perseverance and constancy agree as to end, since it belongs to both to persist firmly in some good: but they differ as to those things which make it difficult to persist in good. Because the virtue of perseverance properly makes man persist firmly in good, against the difficulty that arises from the very continuance of the act: whereas constancy makes him persist firmly in good against difficulties arising from any other external hindrances. Hence perseverance takes precedence of constancy as a part of fortitude, because the difficulty arising from continuance of action is more intrinsic to the act of virtue than that which arises from external obstacles.
Reply to Objection 1. External obstacles to persistence in good are especially those which cause sorrow. Now patience is about sorrow, as stated above (Question 136, Article 1). Hence constancy agrees with perseverance as to end: while it agrees with patience as to those things which occasion difficulty. Now the end is of most account: wherefore constancy pertains to perseverance rather than to patience.
Reply to Objection 2. It is more difficult to persist in great deeds: yet in little or ordinary deeds, it is difficult to persist for any length of time, if not on account of the greatness of the deed which magnificence considers, yet from its very continuance which perseverance regards. Hence constancy may pertain to both.
Reply to Objection 3. Constancy pertains to perseverance in so far as it has something in common with it: but it is not the same thing in the point of their difference, as stated in the Article.
Source: Summa Theologica (New Advent)