Good question.
I’m “old and sick” just like you, well, maybe not quite like you but 68 and suffering from deadly COPD.
I’ve scratched at 3k (or was it 4k) ~5 years ago (and not ~3 years ago like I recently wrote somewhere here) … and after that have fallen back to <gasp>meanwhile 13k!
So … no, it definitely is NOT ALWAYS FUN, rather the opposite, it can be, at least for me, quite painful sometimes.
I mean, losing a stone in strength – or three, if you will – can happen, and I was used to “two steps forward, one step backward”, which, as I felt, made for a quite stable stance at whatever rank I had in the past, and for solid, if slow, progress. But losing ten stones in five years is quite worse than that, right?
Possible reasons:
-
getting old and slow
-
lack of oxygen in brain due to the COPD (I quit smoking 15 months ago but the disease doesn’t care and progresses mercilessly)
-
depression
-
lots of other priorities – i.e. no study, no reviews, etc., only a few Tsumego off and on
-
and the folks I play with are younger and most still have ambitions in Go, i.e. many of them study and review.
Anyway, whatever it is, it IS, and I must try to live with it, like, “what is, is, um what ain’t, ain’t”, right?
A few times I was close to quitting because it was just too frustrating, but then I always caught myself again (do you use this idiom also in English?) and stuck with the game, also because the people I play with (almost exclusively here on OGS, very rarely IRL) are simply so nice persons with whom I like to stay in this friendly sparring contact.
So … is it masochism? I don’t think so; I rather believe that, while YES, it often is painful, it still gives me enough positive feelings, for example when, even in a game that I’m losing, I play a few good moves that steals a few points from the opponent and gives them the creeps 
I don’t know, though, what the future brings. Will I fall back even more? Will I still enjoy it if I fall back to 17k? I cannot tell.
But just like I learned to resign a particular game when I realize that I’m losing it very badly, no chance to turn it around, and when it is simply getting too painful to continue playing, so I might even quit Go as a whole in case that should happen.
But as of yet I cannot see that happen.