I hope something works out for you roxie **sending some hugz**
Unfortunately it isnt unusual for people to end up having no choice but to walk away from things. I was in college when I got this illness and pushed myself there getting worst, I really really thought I could make myself hold on and do it but it turned out I couldnt and I ended up long term bedbound due to that. I really really wish I'd left sooner... the consquences of forcing myself to continue my studies stayed with me for years.
When I was improved some I got some full time work.. a great job for a ME/CFS person in which I also got paid for being there for another while they slept (I was call for a quadruplegic person if he needed anything during the night). While he slept.. I slept too and got paid for it. (Ideal kind of ME job

).
Issue was I ended up having to leave that job as the guy I was doing the carework for, was so verbally abusive and cruel so I had no choice to leave it. The stress he gave (with psychological games and threats) wasnt good for my ME. I didnt regret leaving that job as soon as I ran into major trouble as I didnt really loose anything, as I was then able to just go into unemployment .
I then tried to apply for disability (Im in Australia so maybe things work differently here).. and while my disability application was going on (which keep on being appealed... I was forced to go onto unemployment.. I then got a part time cleaning job.. but unfortunately then developed MCS with the ME. Then unfortunately got sicker and sicker again due to the exertion too of doing part time work. The unemployment department made me keep doing the part time work under threats of they'd otherwise cut of my unemployment payments. Having to keep pushing myself.. my ability kept decreasing until i was only able to work 1-2 hrs per week and was completely unreliable even with that. At that point my specialist .. ordered me to stop completely working due to what it was doing too me and it just made the unemployment dept push me more and more and expect me to be doing more. (I loved my clearing job and my clients).
I think working even a little.. was harmful some to my disability case as it just kept up the expection there of those who kept assessing me of "maybe" i was possible of working (even thou I was doing less then 8hrs per week). I thought it would show them I was really trying and not lazy.. but it just didnt help things.
(So then I was still on unemployment with the fear they were going to cut me off due to their threats).. Fortunately at that point (third appeal) my disability pension finally was approved. I was so ill by this point that I couldnt even sit up at appointments and would have to lay on the floor. My cleaning clients.. they were writing to the gov dept over the fact I couldnt even do my job properly due to being too sick and hence needed to be on disability. (my agency boss was lovely and I felt so guilty as I was constantly letting her down). Disability no matter what country you are in, is usually one big struggle to get but which should end up getting after the long hard battle for it.
I cant advise you whether you should leave your job or not seeing you already have been struggling throu it for 18 years and only have a couple of years left before being able to retire. But what I do advise is to start keeping very good records of what is going on.. eg issues you have at work, where your health is letting you down.
Also make sure you have a good specialist who understands ME/CFS who can back you up when all shit hits the fan with things and who is willing to support you in getting your needs met eg fill out forms, write letters etc etc Having a specialist who is used to dealing with ME/CFS patients and helping them get onto things like disability etc etc could end up being important.
I just hope that what ever decision you make, that it will prove to be the right one.
For me.. I wish I'd left my studies earlier and also wish I'd applied for disability YEARS sooner (I avoided doing that as I knew it would be a struggle to get). I never thou thought the disability process would take so long and I would of never thought I'd be getting knockbacks to disability when I got to the point where I couldnt even sit at appointments.
I'd had a misconception that I'd get it easier the sicker I was when I applied which was actually fase.. the sicker I was.. the less I was able to deal with the whole application and appeal process and hence didnt present my case well (as my brain just couldnt).. if I'd been not as sick and able to think better..I could of presented my case better.