I've been with my fiance for nine years and as much as it sucks, libido is gonna go up and down, and worse, not be in alignment all the time. I've been in your shoes so I know how it feels. My fiance actually just got off of her SSRIs after many years and things have been better than ever. I'm sure you and your boyfriend can get to that point eventually.
In the meantime, or if that doesn't end up happening, here are some thoughts in no particular order.
Sex definitely matters, but it's not the only thing. Try not to lose sight of the other things that are important in a relationship. Try to be extra appreciative of the time you spend together, regular cuddling and other romantic stuff. If you can't have sex, try going on a date instead.
There's nothing wrong with masterbation. You gotta take care of #1
COMMUNICATE! My fiance and I express our love very differently. Talking about it and making sure each of you are on the same page and meeting the other half way is critical. Especially with SSRIs in the mix. It's going to be harder for him to be in the mood so you're going to have to try harder to get him in the mood. And he should be trying hard to, at the very least, make sure you know you are loved.
In my opinion, your romantic partner is basically your best friend+, the exact person you would want to be roommates with. Obviously you want your relationship to feel like more than that, but sometimes that's just how it is. And that's ok. I've definitely heard the just roommates argument more than once in my own relationship. Just communicate and focus on the romantic/non sexual stuff like I said above and you'll get through it.
Therapy is some good shit if you can afford it
Try not to whine. Your sexual needs aren't being met, so it's natural to want to complain about it. It's a turn off and will be counter productive. It's difficult because you are already feeling insecure but frame the conversation like something along the lines of "my needs aren't being met." "Are your needs being met?" And "what can we do to reach a middle ground?" Don't ever say anything like "why don't you like me?" or "are you even still attracted to me?" Don't make him feel guilty.
Good luck op