Why am I still mad at her?

I always thought once you met somebody else you liked your bad/negative feelings would simply fade away. My ex was the first girl I was in love with and when we split up it devastated me. She started dating somebody else a week later which further hurt me. I was so angry/hurt I didn't speak to her for months. Eventually I picked myself up and met a lovely new girl who I have been going out with ever since. She's lovely and every bit as great as my ex and tbh I don't think of her when I'm with my g/f. But whenever I do bump into her I still get angry/upset (I guess that she broke my heart) whereas she just seems the opposite and is always friendly/nice too me. I don't show that I am mad at her but (am usually just a little blunt) am unintersted in conversing with her. I don't know why she still has this effect on me. I'm really happy with my current girlfriend and would never want to get back with my ex so why am I holding so much resentment. Do you ever let it go? I don't see her that often so it's not affecting my life but would like to just be indifferent whenever I do see her!
Why am I still mad at her?
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