Agreed with PurpleHair about someone being in denial of a diagnosis - and especially late teens - early 20s, when everyone is doing so much and a person lacks the life experience to understand what long term effects it may have - and it can just be too much to think about in detail, how much one could actually miss out on, until it actually hits. Did we all say everything to partners at that age we responsibly should have done?
I wonder if he feels guilty about that somewhere, about you getting a different life from the one you signed up for, and has also buried his head in the sand about the guilt and its effects too.
I have a feeling he could do with some therapy. Pain management centres sometimes offer counselling. Given that he can still drive and is able to consider some types of work, it sounds like his resentment is making him miss out on opportunities and life he could still, for the moment, take advantage of, even if it's for a limited time. Wondering what his prognosis is, does he feel a clock ticking on his remaining abilities?
At one point I knew someone who'd been recently diagnosed with psoriatic arthritis - it seemed a particularly horrible condition, she seemed to find ways to manage it but her life changed quite a lot quite fast.
Given the ever decreasing government provision for disabled people,
I would consider it a slightly different matter leaving a partner who has a disability from leaving a healthy person. (Similar to another current thread I'd think it right to try and negotiate and arrange more around things where possible live with unconventional arrangements, to try and get some space without leaving so readily). However it sounds as if your partner may have enough to live on via this company pension which makes it a somewhat different matter.
Seeing news stories about disability benefits problems - there but for the grace of god/my partner go I, worrying if he will end up like that one day - might also be a source of stress and anxiety to your partner which gets bottled up in his avoidant ways of coping with stress.