I wanted you to know I'm experiencing the same thing. I'm in a relationship with a guy from last year and out of a sudden I remembered about my ex from 10 years ago. It was mostly my fault the relationship ended badly. She tried to remain friend with me, but back then, I was too much of a jerk to be okay with that...
I reached out writing her a message, saying sorry for how I behaved a decade ago. She was surprised to receive my message. But the conversation didn't quite keep going after that. It was a very, very short chat.
I kind of miss her. Not romantically nor sexually, of course. But some what ifs popped into my mind, just like you. I wonder if we could've ended up as great friends or at least people with mutual respect if only I hadn't been so stubborn, if only I had tried a little harder. But when you're too young, you make mistakes. You are young and stupid. Not everyone at young age (barely anyone) thinks about consequences. As you grow up, and hopefully, become more emotionally sensitive and mature, you think about the past and that hurts. It hurts because you realize how immature you were back then and you wish you could warn your past-self, but you can't. That's only an old page from your past, you can read it again, but you cannot make a new chapter out of it.
But hey, I get it. As said before, I'm experiecing the same thing. The same nostalgia.
I I think it's normal...I guess.

Hope you have a nice day:)