No sex

I've been thinking about this a lot and this isn't about him - it's about me! I'll want sex for about a month or two, and then NOTHING. My poor boyfriend. We've been together for three years and it's always been like this (to the point I warned him before we started actually having sex). I'm not sure if there's something wrong with me, or if this is completely normal, but I'm concerned he thinks I have an issue or that even to the extent that I'm cheating on him. He seems absolutely fine with the fact we're nearly sexless. When I apologize he's very sweet to me, and tells me not to. I shouldn't be concerned right? I just feel so bad.