I cannot promise that I will always choose you.
But that is the problem: even now C does. He came back to Penmere. As he always did.
To put it otherwise. The way this tale has been set up, constructed, suggests that there never may have been a "free choice".
It suggests that whatever M does, or does not do: his presence in any of C's past lives always was the catalyst of C's returning memories. In short, the choice is, methinks, not C's alone, it Marcus'/Marco's, too. It is he who returns to Penmere. With one aim only: to find a new C.
And because C returns - we do not know if he always lives there, or sometimes left, in at least some of his past lives, as seems the case - the wheel starts spinning again.
For though C might have decided not to act upon engaging with the N he meets, not to act upon the memories those meetings trigger, i e. not return to loving M again, nothing suggests that he ever decided to go his own way. Indeed, he is not doing so, now, beyond telling himself, and V and N that this time all is different.