One school of thought on dream interpretation holds that all aspects of a dream refers to the dreamer. In that case, both the girlfriend, the ex, and dream-you all refer to you. The new gf could represent the part of you that loved him; that part of you probably felt anger and resentment when the relationship ended. The new-gf trying to hurt him and dream-you could be an expression of that, esp if you are feeling a bit embarrassed or feel as though you should have known better.
Good luck. You'll find someone who is a way better match for you!