How to forget about an ex when you are still in love with him?

We broke up about 8 years ago then I started dating my current boyfriend. Everything has been fine until about a year ago. My ex sends me a message on FB and apologizes for the past. We continue talking the whole day via fb, then we exchange numbers. We have been talking ever since. He tells me he is still in love with me and never will stop. Here is the problem, he is married! I know I am in love with my ex but I don't want to be a mistress or a home wrecker... How do I move on