In my case it has depended on the relationship and how close we were. For all of them except my first boyfriend it doesn’t bother me at all I’m glad to say. But after I broke up with my first boyfriend he was devastated when I was with someone else and heard about it and I was the same with him. I was absolutely distraught when he moved on and am still weird about his new gf’s after me.
For other Ex’s, it doesn’t bother me one iota.
It was hard because I always regretted breaking up with him as I was young at the time and since have missed him not being in my life as much.
I think there can be so many reasons for still feeling bad from more self focussed issues (eg) pride, competition, lowered self esteem, regret, feeling cheated or a victim and not letting go…..to more relational reasons (eg) grief for the loss of that person in your life.