How does one move on after heartbreak?

I had a relationship with a girl that even thought did not last long, I deeply cared for her. Unfortunately things ended and I found out I got played. She had chosen to go back to her ex, and never told me a thing. She just had me on the side for long. When I found out by other means I cut ties completely with her, and since then I have not talked with her, and I am not planning to do so. Problem is, since then I have been feeling like I lost myself, or even like I am a loser. I feel like I became bitter and that now I hold a resentment towards her. This only holds me to the past, and blocks me from having something meaningful with someone else and moving on. How does one move on from something like this? How can I start thinking in a different way, and not be bothered by the fact she is doing well and that I am struggling with who I am.