Forget how to have sex???¿¿¿

So this is something very personal, but I don't have anyone to talk about it with. My boyfriend and I have been together for almost four years, next month is actually our anniversary. We're 24 and 25 now and started dating back in college. In the beginning we had sex fairly regularly. Over the years it's gotten more sparse. Every week, every two weeks, every month, every couple months....now we haven't had sex since Halloween. He's never been a super sexual person and more often than not I felt like he did it solely because I wanted to. I'm a pretty sexual person. I've tried to not worry about it but lately we've half ass started trying to arouse each other the past two days but it's SO awkward. We will make out then just stare at each other as if we're trying to decipher what the other person wants to do. I love my boyfriend a lot, I fully hope that he is the man I marry but I'm at a complete loss on why we don't have sex anymore and why we can't seem to get back in the groove of it. Has anyone else had this problem? Does anyone have any advice?