Boyfriend always too tired for sex

Confused about my bf being too tired all the time. Our sex life is always up and down. I am always the one lately that tries to initiate& I'm tired of it because half the time I do it goes no where and he says he's not in the mood, then I feel like annoying. It makes me upset . Not sure what to do and what this means but it's not fair that the only time he wants to do things is when he wants to and not when I do. :/