“I really and truly liked this guy that works at a pub. He had built my hopes up that we would be together because he is in a very unhappy relationship. He kept giving me signals but I have found out by a friend that he’s not interested. I treated him well and he threw it all away. How can a man do this? Why couldn’t he say he wanted us to be just friends and not send me the mixed signals? He was the best thing in my life and I had no warning of him turning me down.”
Jennie, Nebraska

The Advice
It is almost impossible to start a new and lasting relationship with someone who is already involved with another. The situation is always too complicated and there are just too many obstacles to overcome. You have to remember that it takes two to make a relationship and the love or infatuation of one cannot ever be enough to make the relationship work.
Even if he did leave his partner for you, there are still a multitude of obstacles. Such as the feeling that he might leave you for another just like he did in the past. There are also rebound issues, like his continued affection towards his past partner. Do you really want him to be with you while he is thinking about another. This is the standard transfer of affection associated with all rebounds.
My advise  to you is to just move on. You will find someone who is not in a current relationship that has all the great characteristics of this guy but without all the emotional baggage.