I'm struggling with guilt over starting a new relationship with a friend's ex girlfriend and not sure what to do.

My friend, let's call him J, dated a woman M for a year and a half. I knew J for several years, but became close to both of them over the last year. I really respect and like them both and we had a lot of fun times.

J moved to California in August and M was planning to move with him once she secured a job. They broke up around Thanksgiving which I was really sad to hear. The breakup was caused by J getting cold feet M was understandably heartbroken. I began seeing a lot of M and developed feelings for her quickly. I swear these feelings were NOT there while J and M were dating. Two weeks after they broke up I confessed to having a crush on her. M initially rebuffed me, but I was so smitten I couldn't help myself. I really think she's someone special.

Eventually she changed her mind and we started dating around New Years. I really, really like her as a person. I'm not sure about the potential but I do like her. M told J that we were dating around this time and he said he was very hurt. He said he felt betrayed by me, that he still loves M and wants her back. M seems to want to be with me, but at times she seems conflicted too.

I don't want to lose this chance with her but I also don't know what I owe my friend. We're all in our mid-30s and should know better than to do this kind of messy thing, but here we are. I fear I am just a rebound or worse getting in the way of the two of them reconciling. What should I do?