Aquinas says that without grace we can avoid each individual mortal sin but not all mortal sin, at least not for a significant length of time.
On its face, this seems contradictory. After all, I can avoid the first mortal sin. If I avoid it, then I should be able to avoid the next one. And so on. And hence I should be able to avoid all mortal sin.
But this argument mistakenly agglomerates what one can do. For instance, suppose that there is a mine field with a thousand mines. I know how to defuse a mine, but I have an independent probability of 10% of slipping and detonating the mine. It's correct to say that each mine can be defused by me—being able to do this with 90% reliability is sufficient for this—but it is incorrect to say that I can defuse the whole minefield. It is appropriate to look at the minefield that one is to defuse and think: "This is an impossible task without help."
The person without grace is unable to presently control her future actions in favor of the good. Each individual action is in her power, but she cannot control them all at once. Hence she can rightly look with trepidation at her future moral life and say: "This is an impossible task without help."
But a virtuous person can control future actions in favor of the good en masse, by growing in virtue and resolving to do good.