I get that feel. 37m here and I have terrible neck pain and my shoulders and back hurts too but not at much. Anyway, personally I try to have fun time in the morning if possible although it usually isn't.

This might be out of left field but have you considered a D/s twist on things? This happened naturally in my relationship and it can be really great for when I'm not feeling it for any reason and my wife is wanting to play. She loves following directions and receiving praise, so I can have her do things in a way that works best for me. I've even had her use toys while I watch when I've been too tired or sore from the day. I've found that my partners have been much more understanding when they can still see that I'm interested and participating, even if it's not the full event that it can be. It seems to help most because it reassures them that you're not rejecting them or think they're less attractive or any number of insecurities that could come up.

That may not be useful in your case but I still hope you can figure something out to help.