Wow, that is pretty strange...
Well if I try and put myself in her situation...
Suddenly saying to someone that I'd spent a year and a half with I have to try and stop loving you, to me that indicates a feeling of guilt and a reason for her to dislike herself being with you.
If she is seeing, or saw another guy, this could be the key. Maybe being in love with you, so deeply in love with you that she doubts that she could make you happy, makes her feel insecure in some way.
This guy, might give her freedom, because she is not so deeply attached to him. It is indeed a very strange situation that you're stuck in.
If she says she's happy when she's with you, then maybe that's good, but is she happier when she's away from you?
I can't even imagine how confused you must be feeling right now.
[.. I shall call him squishy, and he shall be mine, and he shall be my squishy! ..]