Falling for somebody who isn’t yours

Can somebody help me like give me advice I’ve been liking this dude for 7 months he has a girlfriend. And it’s the worst feeling in the world because I wanna get over him he keeps hitting me up because he just wants to fuck me and I want to but I don’t want too bc he has a girl and I fuckin like him I feel like I will never move on or get out of this situation I feel like I will always have a weak spot for him :/