I agree with Himster29 on most points. I think I may have some additional questions to ask which I really hope may help you pinpoint some other possibilities.
Are there any medications involved for depression?
A friend of mine was diagnosed with severe depression. She was given medication to help with the depression. She didn’t have the added condition of ADHD, but she did have a boyfriend who was used to them having intimate times at least once a week. On the medication, her libido vanished completely. She complained about it to me once after class, and I looked up the side effects of her medication online–she didn’t at the time realize you could do that.
We found that the most common complaint after dry mouth and nausea was…a severe lack of libido. Her medicine was FAMOUS for it.
She saw her doctor, got on a different medication for the depression, and her libido returned.
Depression itself can also cause a lack of libido, medicated or not.
During your discussions, has any shame been brought up?
My spouse and I when we were dating had an issue where he felt he wasn’t performing well enough for me to be satisfied. Turned out that caused him a lot of shame which of course affected his libido. I wasn’t responding the way he was used to–and that was due to work stresses on my end. Oops. We didn’t just talk about it–I started to try to find ways to de-stress from work and to make him feel wanted and loved in other ways. One way was just making it a point to kiss him passionately once per day. Another was to text him suggestive things. It’s hard to feel not-good-enough when your partner is clamouring to be with you.
Is it possible that this is partly a thing about feeling like he knows everything about his partner in the bedroom?
My brother used to get “meh” about sex with his long time girlfriend (later wife) due to boredom. He knew her, what she liked, he knew what she liked, and there was no discovery/no chase–no stimulus. For a while their relationship was also sexless because of the no thrill of the chase thing. He was ADHD/ODD, and ended up having multiple affairs. Some his wife didn’t know about, but I learned about later. At least one ended up in a child outside of their marriage. They ended up arguing a lot which brought some heat back to things. (I am NOT advocating conflict as an approach to better or more frequent sex–and if I did, heaven forbid, I suspect it may turn you off. You don’t strike me as the type to want conflict, from what you’ve written. 🙂 I’m also not advocating for opening your marriage if that’s not your thing.)
I know these aren’t answers–it’s just more questions–but I’m hoping if more questions are brought up, perhaps more can be discovered, then maybe a solution can be found. I seriously sympathise with your situation. Been There, Done That–more than once.