Boyfriend rarely wants sex

My bf and I have been together for about 6 years now, we’re both in our early 20’s so I get that life is kinda chaotic at this age and sex obviously slows down. However, we’ve gone from having sex a few times a month down to like once a month if even. Recently he’s turned me down so many times and it hurts me so much feeling like he’s rejecting me. I’ve talked to him about it and he can never really give me a reason, most of the times it’s because he is “too tired” or not in the mood. (I totally get that and I respect his decision). He never even makes out with me because he thinks it has to lead to sex, whether that’s the real reason he doesn’t do it, i honestly don’t know. He kisses me when we say hello and goodbye, not much more than that unless we do have sex. I feel like we are totally lacking in the sexual department and I’m not feeling sexually fulfilled in anyway whatsoever and honestly it’s really frustrating. I don’t turn him down unless I’m not feeling good, as his girlfriend if he’s wants sex we’ll do it because I know what it feels like to want it and not get anything in return. I get ready for him and it sucks hoping it may happen just to be let down yet again. I’m feeling very upset and sad, it genuinely makes my anxiety so bad. My self esteem plummets, after constantly being rejected it makes me feel so bad about myself and my want for sex just disappears when he makes me feel this way. I feel like this is causing big issues between us, it’s almost driving a wedge between him and I. Please no rude or bitchy comments, I just need some real advice on what you would do in this situation from girl to girl😭