Originally Posted by JinnKai

Since you've already decided to the conventionally "right" thing, I won't berate you with the ethics of the situation.

However;

You might have ruined your chances with this girl if you tell her now. You did ruin your chance with this girl if you tell her later. So maybe you can't be certain that you didn't ruin it, but you can be certain that your odds are better now than later.