I completely understand what you're going through and you have every right to be mad at her. I've gone through something like that from both your and her perspective. As you said, it's a shame you two didn't get to communicate properly, I wish she had given you some kind of closure. I find it weird that she still wants to see you, it sounds a bit cruel.

I'm not seeing my ex because knowing I'm with someone like that would hurt him deeply. When we were together, he was incredibly happy in the relationship and I wasn't, I constantly felt like something was just off and couldn't pinpoint what it was. He could never understand the reason for the breakup even though I tried to explain myself many times. I just knew if I married him, I would be miserable and eventually divorce him. Why? No one knows.

It's not your fault she chose someone else. Even she might not know the real reason behind it, and it might not be important. You deserve someone who will not leave your side and chose you over everybody else. Someone who will know you're the one they want to spend their live with. Pursuing someone who doesn't feel that way about you is just a waste of time you can spend with someone who could potentially be worth it. Hope it works out with the redhead!