Addressing the question whether Christ's grace as Head of the Church (his "capital grace," if I understand Aquinas aright) is distinct from his personal grace (that grace enjoyed by his human nature), St. Thomas affirms that there is no such distinction:
It is written (John 1:16): "Of His fulness we all have received." Now He is our Head, inasmuch as we receive from Him. Therefore He is our Head, inasmuch as He has the fulness of grace. Now He had the fulness of grace, inasmuch as personal grace was in Him in its perfection, as was said above (Question 7, Article 9). Hence His capital and personal grace are not distinct.Now the point that I am making really has nothing to do with the question Aquinas has posed for himself here. For purposes of this post, what is relevant is that he says that it's Christ's grace that justifies others. Hence they do not justify themselves; we do not justify ourselves.
I answer that, Since everything acts inasmuch as it is a being in act, it must be the same act whereby it is in act and whereby it acts, as it is the same heat whereby fire is hot and whereby it heats. Yet not every act whereby anything is in act suffices for its being the principle of acting upon others. For since the agent is nobler than the patient, as Augustine says (Gen. ad lit. xii, 16) and the Philosopher (De Anima iii, 19), the agent must act on others by reason of a certain pre-eminence. Now it was said above (1; 7, 9) grace was received by the soul of Christ in the highest way; and therefore from this pre-eminence of grace which He received, it is from Him that this grace is bestowed on others--and this belongs to the nature of head. Hence the personal grace, whereby the soul of Christ is justified, is essentially the same as His grace, as He is the Head of the Church, and justifies others; but there is a distinction of reason between them. [ST III, Q8, A5]