Hi zana66. Even though it's been awhile since you posted I'll jump in here. Reading your post, it seems clear to me why it bothers you. You mention having low self-esteem. I have always had the same problem regarding the way I view myself in relation to others and it has always been a problem in relationships and friendships. I was never able to just relax and trust anyone or anything because in my mind I was always being compared to someone else and falling short. It's taken me a long time to realize that the only person doing this is me.
I am married now, and have been with my wife for about two and a half years. The first year of our relationship was a nightmare. I had taken four years before that to be single and figure things out, because every relationship I'd had up until then (I was 28 at the time I went celibate/single) was a complete nightmare. I was constantly jealous and worried and sick with the thoughts that raced through my mind about my now-exes and all the girls they'd been with and everything else that went through my mind: they were better, prettier, thinner, richer, younger, more exciting, etc. But as I worked through the first year of my marriage I realized that just because I always felt and thought this way, didn't mean it was true. I was now (am now) in a relationship with a wonderful, caring person who loves me and shows me all the time, doesn't just say it, but anytime she brings up a past relationship or I notice her checking someone else out I lose it.
I react when I am being threatened. The things that threaten me are anything that trigger my fears of abandonment and rejection. What I've had to do is realize that just because my partner has exes, regardless of what they look like or anything else, doesn't mean that my partner cares any less about me. I mean, putting it in perspective, I had a life before her. I had other partners. I was intimate with other people. So why am I bothered about her life? Of course, I knew all that logically, but I couldn't figure out how to get from the head to the heart.
I can't tell you what to do about it, other than to start being aware of when it happens and what exactly the thoughts and feelings are. Are you afraid those other people were more exciting? Better than you? Are you afraid he'll leave you eventually in search of one of them, or someone 'better' than you? Those are powerful motivators. Most people have those fears. I hope you have been able to find some peace with some of this and that things have gotten easier to deal with. Hang in there. It is bloody hard trying to learn a new way of looking at myself after a lifetime of always thinking I'm less than everyone else.