I can’t stop thinking about my partners exes and it’s driving me nuts

My partner and I have been together for a while and exes have been brought up because of course they are a part of who we are as people to an extent. Those relationships will mold people into who they are and what they somewhat want from other people. This still made me feel a little threatened at times for which I don’t feel like giving explanations as for why or specific details regarding what made me feel that way in the first place. A while back I found old love letters in my partners car from my partner to an ex and it’s absolutely crushed me ever since and I can’t stop thinking about specific details regarding them that make me nearly want to vomit. I know I shouldn’t have read them. I just don’t know how to stop being bothered by it and it’s making me lose my mind a little lol. If anyone knows how I’m supposed to not feel threatened or icked out by the thought of my partners exes please let me know cause I’m kind of at a loss and I hate feeling like this.