My boyfriend won't have sex with me

I've been with my boyfriend for over five years. My sex drive has always been higher than his, but it keeps getting worse as time goes on. I would have sex every day, given the opportunity, but it only ends up happening like 2 or 3 times a month.

I've tried talking to him to see if there's something else going on with him. I've tried compromising so that we could at least do once a week. I've explained to him how it makes me feel when I'm constantly horny and feing rejected. Nothing has worked. At this point, it's been about three weeks since we last had sex.

I dont know what to do. I love him, but the thought of spending the rest of my life feeling this way is depressing and literally brings me to tears. Should we break up? Try an open relationship? I'm tired of feeling undesirable.