What do you do when your drive is higher than your boyfriend's?

I've been with my boyfriend for two years, we live together and have an otherwise fantastic relationship - he's my best friend as well as my partner. The only issue is our life, which in the last 6 to 8 months has slowed down considerably. It's still pretty great, not quite as electrifying as before, but he's the best lover I've ever been with. All that said, we're now down to only having 1 or 2 times a week, and I find that's just not enough for me (I know this is common enough in other relationships, but you can only measure your own by your own standards, right?) I'd prefer it to be more like 5 or 6 times a week, if not more, but so often these days when I try to initiate my partner turns away, or falls asleep, or openly says he's not in the mood. We spend so much time together I'm pretty much sure he's not cheating. I find myself stealing opportunities to masturbate when he's not around and almost feel like I'm cheating. Help! How do I deal with this?