I did my best friend wrong

I lived with my best friend Tara and her boyfriend Alex. Before we all moved in together Alex and I started doing thing behind her back. I thought I was so in love with him I would do anything. It went on for 2 years and after that I had to tell her because I couldn't handle it any more. They broke up and now she has moved on with a great guy who loves her and treats her like a queen. Alex was mad because I promised I wouldn't tell her or anyone. 2 months later and Alex is wanting to talk to me and I guess date me but I don't know what to do because I'm scared he'll also cheat onme ........... Also forgot to mention that she didn't love him. As her best friend she would tell me she didn't know how to leave him because she was attached to having someone to sleep with at night. She would also treat him like crap and call him worthless and a piece of shit. That's when Alex started talking to me about his feelings and I would listen then talking turned into something else. even after he would let her stay home and he would work and buy her whatever she wanted she would still treat him like crap.