we don't have sex anymore?

So my boyfriend and I have been dating for 4 years and until a few months ago we had a really great sex life. but then it kinda just.. idk, stopped? we still hold hands and stuff, like he's still affectionate but just no sex? And I'll be really obvious about wanting it but it's like he doesn't even notice. I've brought it up once before and he just says that he doesn't notice that I'm going for it, or he's tired.. He's 20 and I'm 18 and I know this is just kinda what

happens when people date for a long time but if it's unchangeable, I kinda wanna break up with him.. my sex drive is to high and I'm to young to hang up the towel..