I'm the same as OP. My GF is not the jealous type at all but I am and I hate myself for that. I was a virgin before I had sex with her but she had one (only one) partner before me (they were more like FWBs). I want to have a much higher body count than her so I asked her to have an open relationship and she agreed to it happily even though she said she wasn't interested in it and only wanted me but had no issues if I wanted to fuck other girls. The only thing I'd need to do is let her know about it.

I also can't get over her ex FWB. Although, she's told me I'm much better than him in every possible way, it bugs me that he was wayyy taller and had a more muscular body than me. Plus the things he did with her in the bedroom that I haven't already done with her even though she has told me that I'm hundred times a better lover.