Relationship insecurities

New to the forum looking for some good advice before I destroy another relationship. For the last year and a half I've been seeing a wonderful woman. We have been seeing each other discreetly and just recently she has left her husband, the two of us jointly deciding to become a couple. Seperation agreements are in place. That part is suprisingly stable. I'm the one with the problem. We have spent a great deal of time talking to one another in depth, our like and dislikes etc. But just the other day we were talking about sexual interests and I mentioned threesomes. I personally went through a phase where I thought that sounded like something I would like to try but really the thought doesn't do much for me. I'd rather be exclusive with my girlfriend. The suprise was she said she had already been in one 20 plus years ago in high school. Now I can't get that out of my head. I feel like I've been cheated on, my insecurity is insane. My last relationship was about a year and a half ago (slight overlap of girlfriends) That relationship lasted 14 years but I almost destroyed that one too for much the same reason. When I met my last girlfriend (15 years ago) she told me about the number of partners she had been with. That just about destroyed me for the same reasons. What's wrong with me?

You have to stop shitting in your own garden! Every time you have those jealous thoughts, just remember that she left her husband to be with YOU. Say that like a little prayer every time you feel threatened. But of course it is more complicated than just that, overcoming insecurities is a lifelong journey. Think about where it all started - was it something that was done to you by a girlfriend, a hang up you have from witnessing messed up relationships in childhood or is it just your own feelings of wanting other women (that is common and healthy and quite okay) that are making you project your resentment towards your lady?