I need advice

Ok so something has been bothering me so much lately and it’s causing me to go into weird depressive episodes but it’s so weird. I’m 18 and just started dating this guy and he’s amazing but for some reason i have been obsessing over the fact that he’s had sex before me and I have too which is the strange part but it’s on my mind probably 60% of every day and he doesn’t know but i keep making scenarios in my head of him having sex w other girls and every time me and him have sex that’s all i can think about! HELP