What we merit, we obtain from God, unless it is hindered by sin. Now many have meritorious works, who do not obtain perseverance; nor can it be urged that this takes place because of the impediment of sin, since sin itself is opposed to perseverance; and thus if anyone were to merit perseverance, God would not permit him to fall into sin. Hence perseverance does not come under merit. [ST I-II, Q114, A9]
That is the sed contra for article 9. Normally St. Thomas reserves it for an appeal to authority, which ordinarily comes from Scripture, the Fathers, or some recognized authority; in this case he simply says it's unreasonable to suggest that we could merit perseverance.
But that is not all.
Since man's free-will is naturally flexible towards good and evil, there are two ways of obtaining from God perseverance in good: first, inasmuch as free-will is determined to good by consummate grace, which will be in glory; secondly, on the part of the Divine motion, which inclines man to good unto the end. Now as explained above (6,7,8), that which is related as a term to the free-will's movement directed to God the mover, falls under human merit; and not what is related to the aforesaid movement as principle. Hence it is clear that the perseverance of glory which is the term of the aforesaid movement falls under merit; but perseverance of the wayfarer does not fall under merit, since it depends solely on the Divine motion, which is the principle of all merit. Now God freely bestows the good of perseverance, on whomsoever He bestows it. [ibid.; emphasis added]
In other words: perseverance is a gift of grace as well. This is why we pray that we might persevere to the end – because we hope to receive that which we do not deserve [see ibid., ad 1].