Being irrationally jealous over sex in my head

My partner and I have been together for a couple of years now, but at the time being we are not living in the same country. Before dating me, my partner was in a long-term relationship with person A, whom I knew, as before dating, my partner and I were friends. Eventually, when we started dating, we also started having sex, and all of a sudden I found myself being awfully jealous over practically nothing. Whenever I think about my partner and I having sex, I unintentionally picture them having sex with person A instead of me. These visions are absolutely hideous, nearly disgusting and an instant turn off, and I don't know how to make them go away. They almost haunt me, every time I think of something sex related, they come up, and stay for hours and hours. I've never told my partner about this. I still have some time to cope with this on my own before my partner comes over, as I don't want to ruin our sex life with my stupid jealousy or whatever this feeling is. This issue has been with me for quite a while now, and I feel that I can't cope with it on my own. Simply 'letting it go' doesn't seem to help.

Hey Susiedqq. That's exactly the thing - nothing good comes from this thought. That's why I'm trying to get rid of it. I have a lot going on in my life actually, so it's not about time really. I come home late and rarely have the time to just brood in my misery. It just pops ups every now and then when my partner and I chat about things. It's just that this relationship is the best thing that ever happened to me and I don't want to lose it. On a side note, my partner and I are both female. Her ex is male (she is bisexual). And that makes me cringe.