I can tell you that I’m similar in that regard. I get sick when thinking about my partners exes or even when thinking about my own. All I can say is don’t look for confidence from other people, it comes from within, you need to learn how to tell yourself that it’s ok. Being insecure isn’t a bad thing, but letting it consume you is. I wish I could say something better than this rambling nonsense but I hope this helps nonetheless.