I'm not a massively jealous person anymore, but I used to deal with retroactive jealousy, aka jealousy about a person's past. It was wild.
I had it BAD during my first serious relationship, he had had a previous serious relationship of a year or two, I had had a few boyfriends of 3-8m so I was really burned up by the idea that he had loved someone else, been intimate with them, what they'd shared. Bear in mind we were late teens so everything is more intense.
It was so bad the smallest thing would set me off, a lot of my life was burned up with feeling sick at the thought of him having been with someone else before me. I felt like I hated the girl. I would ask him questions like if you walked past her house on fire would you stop and help. Once we were watching a film and I asked if he'd seen it before and he said yes and it was with his ex and I couldn't continue watching. We once went bowling with friends and one of his mentioned the time something funny had happened when they'd been with his ex and I left, it hurt me that much. I would resent people who shared the same name as her. If a character on TV had her name, I would feel sick and have to turn it off.
It was wholly irrational and incredibly destructive, it caused me so much misery.
Once I grew a bit older and had more relationships the retroactive jealousy totally dissipated. I think it was once I'd got some serious relationships under my belt I didn't feel inferior anymore as I had an equal past if that makes sense? One of my boyfriends had slept with 100+ women before me and it didn't bother me in the slightest, we'd run into an ex FWB out and about and they'd say hello and hug and I'd just be like oh nice to meet you. My husband obviously had exes before me and I feel nothing about it, if we ran into one I'm sure he'd stop and say hello like we have when we bump into an ex of mine and have a chitchat and move on. Having said that DH has major retroactive jealousy and I feel for him as I know what it's like. He would at times vomit when we started dating thinking about me with another man before him. To this day I have no contact with exes or even friends I slept with briefly because of the impact on him. It's such a destructive feeling and you can certainly work on it (we have psychology books and all sorts) but it's hard to shift.