You could be writing about a close friend of mine and her DP.
Diabetes type 1
Head in sand approach
Initial enthusiasm about sex, now waned away to zero
Wasn't managing his glucose levels properly
Diet was shit
Kept "forgetting" about his medical appointments
Appeals to reason, logic or emotion all failed
This led to a period of about 3 years where he was having hypo seizures - at one point he was being hospitalised on an almost weekly basis.
The seizures have caused brain damage over the years and he has major problems now with short term memory loss, ability to comprehend things like instruction manuals, loss of agility (meaning he had to give up his much loved hobby.)
It got to the point where my friend gave an ultimatum. Either start taking care of himself, and properly, or she would walk away.
It took a long time (many years) to get his diabetes under control. They never resumed sex and instead after many years chose to open the marriage. She has sex with several other partners. He has the freedom to do so if he wanted, but doesn't.
In the interim before opening the marriage, she spent years thinking of herself as repulsive, ugly, fat, totally repugnant. She was quite astonished that when she did start to date she had men and women throwing themselves at her!
It works for them but I would have been out the door many years ago.