Terrible sex

My boyfriend is such a great guy in every aspect except one, his sex sucks. So much so to where I'll fantasize about my ex (who's sex was the best I've ever had) while with my boyfriend. How do I tell him that he's not satisfying me sexually? Is there a way to fix this? I don't wanna be hung up on sex but I'm a horny girl. Is this worth breaking up over? Should I find an alternative to save our relationship??