Several months ago, I started a relationship with my boyfriend and everything was "normal" in terms of sex drive and whatnot. Then it died down to zero for him.
He told me about his past, his 10+ years of being on Adderall, his depression, and his many failed relationships before (including several years without being with a woman sexually since).
It's just an overall apathy to the concept of sex and intimacy for him, and it's caused him even more stress that it is that way since we are together. I'll admit that it affected me as it started because I assumed it was a lack of interest in me, but he assured me that it wasn't after we started talking about it.
We've been open about the issue and I looked up what I could find about causes for it (the Adderall, depression, and dry spell all are possible causes, not to mention the past relationship issues of guilt and whatnot). It's a combination of not having a desire for sex, but also short duration if sex stuff actually does happen. He also smokes and isn't the best at diet or exercise. I have a feeling that a lot of it is medicinally-based, and then a heavy layer of psychological issues with insecurity and anxiety on top of that, which I've done my best to reassure him about. He's still able to masturbate, but it's more of a stress relief necessity than actually wanting to or even enjoying it much.
I don't want to break up because I love him for more than just the physical aspect, but I also know that it would not be a viable relationship in the long run given that I do still have a sex drive. I just don't know what can be done by either myself or modern medicine to help the situation.