Wish I could get this out of my head

My ex left me for his ex a few months ago and it crushed me. He always talked about her and how much he hated her and how mean she was. She would call and text him 24/7 and I tried to be nice and let him handle the situation but he never did and now I realize its because he wasnt over her. I really fell for him and he lied to me the entire time. She hurt him and she gets to be with him now and they seem happy. Does karma even exist ? How is it fair that they get to be happy together after destroying ever ounce of confidence and happiness I had. I have so many ugly thoughts in my head and I know I should wish them the best but I feel so wronged. He told me he wanted me and only me. And I fell for it.