First of all, you need to identify what exactly is wrong. Perhas you have done that already and do not want to share the details (which is perfectly understandable and absolutely fine). However, I am reading it as you have so many unknowns that it is causing undue stress, which only addds to the feelings of exhaustion.
That being said, once you find out what, if anything, is wrong you need to get as much info about the condition as possible.
I went through renal failure and a kidney transplant. I was unable to do everything I could do before I got sick, but worked y tail off to do as much as I could. I honestly feel that by forcing myself to continue doing as much as I could, it made my actual recovery from the surgery that much better. I was riding withing 5 weeks (much to the sppecialists dismay) and did my fiorst show at 8 weeks out. granted, it was only a 2 - ftt class but for me it was a huge victory.
Once the illness is identified, ask if pushing yourself physically is a good thng or not. I was told to do as much as ppossible becasue the stronger I kept my body, the better. I was riding with a creatinine of 11 and still not on dialysis. Once I started dialysis i was actually better off but they were nervous about me riding with the PIK line in. I learned that I could do a lot - just not as quickly. I had to rest and take frequent breaks, which made everything take a long time, but it eventually got done. I also learned to accept help. If someone offered, for example, to go get the boys fromt he filed to save me the 1/2 acre walk, I acceted:)