This would most definitely be the end of the relationship for me. It doesn't matter how long ago it happened. Actually, the fact that she could live with herself after doing that to you & keep it a secret for so long makes it even more disgusting.

I feel for you, especially when you wrote about wasting so much time in your room talking to her.. devoting all your time to her while she was betraying you. Been there, unfortunately.

Please leave her, you'll find someone so much better.