What can I do about the fear of them being "fixed" or "better" for the next person?

This is my number one lasting fear. The one that always sticks out more. I don't really care as much about if this guy is more attractive than me. Somehow, despite being monkey branched, I still have some semblance of self-confidence and don't always think I'm ugly or disgusting or unlovable.

What really hurts most is the idea that it was because of me. I'll be honest: it was my first relationship. I did things I regret. Nothing like cheating or abuse, but I didn't understand how you treat a partner. I hid things from her about me that I was afraid she wouldn't like, like my nerdier interests that she didn't have in common with me. I would sometimes choose to play videogames instead of spend time with her, and I'd even say I was busy when I was just playing games. And I talked to my friends about the negative experiences I had with her, which sort of pitted them against her before they even met her. But I learned, and I grew, and I stopped doing those things. But she never let go. It was really what ended our relationship and pushed her away, according to her.

That really leads into my main fear. If this new guy doesn't do those things, what if she really is better? What if she is the her that I know she could be? What if all she needed was fewer initial mistakes to build the case off of? Less things to constantly ruminate about? What if she is able to stabilize herself for him? Or, what if he is more patient and understanding than I was? What if he knows how to be there for her like she needs and I didn't? What if I just did it wrong and she just needed to be treated right? What if she does things for him, she never could for me? What if she forgives him? What if she lets him do things she doesn't like because he does other things better than I did? This is where all my self-confidence falls apart. Because I don't think I'm unattractive, or unlovable. But it feels like she did. She said, "you aren't worth my effort and he is". And as someone who basically centered my self-worth around her opinion for the last two years, that still hurts.

I guess I'm just looking for reassurance that she won't change. That even if he doesn't mess up as monumentally as I did, he will make some small mistake and she will still pick him apart. That neither of them will be happy. That it wasn't just my fault.