Your ex sounds narcissistic and abusive, don't fall for her mind games. She said these things to hurt you not because they are true. She wants you to feel bad because she can't see happy people.
You and everyone else has the right to be happy. You deserve happiness and she deserves shit. Don't try to talk to her again asking why she ended the relationship. She gets off on that.
Focus on yourself and your feelings, if thinking of her hurts then don't do it. You don't owe this person anything.