Because of the distance

I always said I would never cheat. I broke that vow recently.
My girlfriend or whatever we are now moved across the country a few months ago. Despite our phone conversations, I’ve been pretty lonely. In the meantime, a very close girl friend of mine has been having troubles with her bf. I’ve known her for years and we have always had a connection but both of us were always in other relationships so we’ve remained close friends. Several nights ago we both got drunk and had (great) sex. I felt so guilty about it the next day, though. I told my girlfriend I wanted to break up, but made up excuses as to why. We still talk but the other girl is sticking around and saying she wants to break up with her bf for me. This ordeal is giving me some serious mixed feelings.