How do you deal when your best friend/ex drops you for someone else?

i dated this person for 2 years and we broke up in December of 2005. We still acted as if we were together, spending the majority of our time together until June 2006 when i started seeing someone else. She took it very badly and felt hurt and betrayed. I dated the other person for about 2 months and things ended. After that, i tried so hard to be close to my ex again and apologize for what i had done. She finally accepted that around October 2006 and again we started spending the majority of our time together like we were still together...again. As if things weren't so gray already, we slept together and made out on several occassions. We never said we were back together though. In April of this year she hooked up with a mutual friend of ours that we had known for several years. Now they are seeing each other. I feel betrayed by both parties and am grieving because i've lost a best friend/ex/love and she feels zero pain at all. She wants nothing at all to do with me. help.How do you deal when your best friend/ex drops you for someone else?
talk to her about how you feel. it sounds like both of you are not communicating with each other so it seems acceptable to see other ppl. you need to understand where the relationship is.........are you just friends or more? otherwise, you two will keep going through this cycle and keep hurting each other which is not healthy for a friendship.