My diagnosis was both depressing (I have a BSc Biochemistry and so knew exactly what it might mean and how it worked) and a HUGE RELIEF (I had had symptoms from age 9 and was diagnosed at age 31 after hundreds of "it is all in your mind" and misdiagnoses (with their mis-prescribed meds) over those years)! Luckily mine is what they call 'benign relapsing remitting'. Benign, not as in 'non-cancerous' but as in 'very slow progressing' and relapsing remitting as in 'some days are good, some are much worse and relapses might take 6 months'.
It explained the 'mono/glandular fever' episode when I was 18 and trying to do my exams but where the mono blood test was negative. It explained the many relapses of 'mono' throughout university (3 year BSc and 1 year MSc) and my working life. It explained the two six-month absences from work due to 'M.E./C.F.S.' and the 'Meniéres Disease' constant dizziness and vertigo that did not respond to the meds (well, we thought that it explained that - this turned out to be vestibular migraines and was cured by taking Propranolol!).
Copy/pasted here for my records and because I need a copy of this to refer back to so that I don't have to type it all out again - I have had to explain it so often! ;-p