Objection 1. It would seem that anyone may merit for himself restoration after a fall. For what a man may justly ask of God, he may justly merit. Now nothing may more justly be besought of God than to be restored after a fall, as Augustine says [Cf. Ennar. i super Ps. lxx.], according to Psalm 70:9: "When my strength shall fail, do not Thou forsake me." Hence a man may merit to be restored after a fall.
Objection 2. Further, a man's works benefit himself more than another. Now a man may, to some extent, merit for another his restoration after a fall, even as his first grace. Much more, therefore, may he merit for himself restoration after a fall.
Objection 3. Further, when a man is once in grace he merits life everlasting by the good works he does, as was shown above (2; 109, 5). Now no one can attain life everlasting unless he is restored by grace. Hence it would seem that he merits for himself restoration.
On the contrary, It is written (Ezekiel 18:24): "If the just man turn himself away from his justice and do iniquity... all his justices which he hath done shall not be remembered." Therefore his previous merits will nowise help him to rise again. Hence no one can merit for himself restoration after a fall.
I answer that, No one can merit for himself restoration after a future fall, either condignly or congruously. He cannot merit for himself condignly, since the reason of this merit depends on the motion of Divine grace, and this motion is interrupted by the subsequent sin; hence all benefits which he afterwards obtains from God, whereby he is restored, do not fall under merit--the motion of the preceding grace not extending to them. Again, congruous merit, whereby one merits the first grace for another, is prevented from having its effect on account of the impediment of sin in the one for whom it is merited. Much more, therefore, is the efficacy of such merit impeded by the obstacle which is in him who merits, and in him for whom it is merited; for both these are in the same person. And therefore a man can nowise merit for himself restoration after a fall.
Reply to Objection 1. The desire whereby we seek for restoration after a fall is called just, and likewise the prayer whereby this restoration is besought is called just, because it tends to justice; and not that it depends on justice by way of merit, but only on mercy.
Reply to Objection 2. Anyone may congruously merit for another his first grace, because there is no impediment (at least, on the part of him who merits), such as is found when anyone recedes from justice after the merit of grace.
Reply to Objection 3. Some have said that no one "absolutely" merits life everlasting except by the act of final grace, but only "conditionally," i.e. if he perseveres. But it is unreasonable to say this, for sometimes the act of the last grace is not more, but less meritorious than preceding acts, on account of the prostration of illness. Hence it must be said that every act of charity merits eternal life absolutely; but by subsequent sin, there arises an impediment to the preceding merit, so that it does not obtain its effect; just as natural causes fail of their effects on account of a supervening impediment.
Source: Summa Theologica (New Advent)