Caramac, although you are doing a huge amount of stuff, I had wondered if it was the diabetes which was making you feel so bad, so am glad that is on its way to improving!
However, there has been some good advice which i hope you are able to take on.
Of course, any of us can see how your childcare obligations place you, and indeed for parents who do not work set hours, childcare can be very difficult.
I am not sure (haven't read all posts in detail) if anyone has suggested that you talk to the gym. A lot are struggling at the moment - they won't advertise flexibility but will sometimes be helpful if you explain - I have twice asked for and been given flexible & helpful arrangements.My own gym (admittedly in a relatively poor area) does 'carnets' so you get a reduction on individual sessions without paying a membership fee.
I also wonder if there are other, more flexible ways of achieving what you want from your gym membership. For instance (and I do see that it wouldn't suit everyone) I now have a 1-to-1 pilates lesson once every 4-6 weeks (£40) to keep me on track and in the meantime,do my own pilates at home.
And I completely agree with those who say FiL must be responsible for himself - although it might be helpful if you explained everything that you have to be responsible for, as in my experience, some 'elderly' (!) people get tunnel vision about the rest of the family!