Why am I having such a hard time forgiving myself?

Sidil
So, long story short, I was in this relationship (long distance) with this girl for about a month more or less and in that time she broke up with me claiming anxiety and something else but I soon found out afterwards through Facebook that she really cheated and saw another guy prior to breaking up. I was so hurt and mad and vowed at the moment that if she contacted me back, I would tell her off. She eventually did contact me (because they broke up) back just claiming she wanted to be friends and I was going to tell her off but something inside me just agreed to being her friend.
So we became friends and she eventually told me about her relationship and I eventually told her I already knew prior to her telling me but decided to forgive her. Fast forward, and she eventually started becoming distant again. After questioning her about it, she told me she was seeing the guy again and had been for a while now but didn't want to stop talking to me and I was a great person and all that and I was extremely hurt and told her to lose my number.

There's a lot of other stuff I left out like how she refused to make the effort to see me when she was in my city and when I was in hers. But I'm just so mad at myself for forgiving her in the first place and letting someone have that much control emotionally over and letting myself be played and mistreated like that. It's just so hard to forgive myself for letting it get as far as it did.
Why am I having such a hard time forgiving myself?
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