How do I (25/F) overcome insecurities about my boyfriend of 1 years (25/M) long time ex of 5 years who broke up with him?

Okay, I'm looking for help with insecurities I'm experiencing regarding my boyfriend's ex. I (25/F) have been with my boyfriend (25/M) for just under a year, and it's the healthiest, happiest relationship I've been in. We are very happy. However, I've been dealing with insecurities about his long-term ex from high school (they were together for over 5 years, and we all went to the same school). I know he was happy in that relationship, and he often refers to those times as some of the best in his life (not specifically with her, but he was close to her family). The hardest part is that she broke up with him, and he only has positive things to say about her. (He doesn't bring her up often, but when we discuss our pasts...) I don't want or expect him to have negative things to say – I actually appreciate it when people don't have negativity surrounding past relationships. But I'm struggling with the thought that if she hadn't ended things, he might have hoped to end up with her. This is a new struggle for me, as I've never had confidence or insecurity issues in relationships before. I'm not sure if it's because I'm serious about my boyfriend, or what's sparking it. I know I need to do some internal work, but I would love any advice because I don't want to bring problems into our relationship. I want to contribute to a healthy relationship and be the best partner I can be. Any advice or words of wisdom would be greatly appreciated.