I like a boy, but...

What do I do? I like this guy, and we've been dating for a whole month, and out of nowhere I get a message from another girl asking if I'm seeing him...

I said yes, and she started telling me that he's been her boyfriend for two years, that he's always cheating on her and everything, but that she always forgives him. I felt bad because I had just slept with him and lost my virginity. I felt dirty because that girl told me everything he did with her, and he was denying me as if I were just a friend. 

I felt so disappointed, I couldn't even cry from the shock, but I love him and I don't know what to do... I feel so stupid and small...