Should I keep on this relationship or let go.

A year and a half ago, a male aquaintance broke up with a girlfriend of 9 years. He went on to date a friend of mine. In January she broke up with him. At the same time I had also just parted from a boyfriend. We got closer and closer sort of helping each other out. As time went on, and one thing led to another, I have fallen very deep for this man. We have become inseparable. He told me he still has a thing for his past girlfriend (the 9 yr relationship) but still continues to want to be with me. She has moved on like any one would with a new boyfriend and even moving in with him. I know he still keeps in touch with herr and I'm okay with that because I know she is commited to her new relationship. What should I do? I know he cares about me very much and we have such a wonderful time together all the time. I feel I can't give up on this but how much can I take. We are both older and time is so important. I really love him and have never felt a connection to someone like this before.