If it's never enough - then it's clearly not appreciated and, yes, other GP's do get this treatment.
Think back to when you had your small children. Did your own Parents live nearby and spend large amounts of their time helping you raise those children? Lucky you if they did, but it seems from the tone of your post that you would have been grateful for that, rather than entitled, and certainly not seek to take advantage.
I can't understand the entitled and demanding attitudes often illustrated in Gransnet threads when it comes to some Grans helping so much with childcare. Wonderful if you help out with your GC's and you're part of a family unit which pulls together and includes you - in good times and bad. And you're loved and appreciated for all that. But, if not, and your own needs, health problems, outings, holidays, time with Husband and friends are dismissed, then you need to have a re-think.
Why not take a step back - and then another one on each occasion that your feelings are dismissed or you feel shunned. You are possibly your own worst enemy : you've gone overboard but can see that it's never enough. Well, that's enough. Stop doing so much and be assertive, firm and unavailable when your own life needs to take precedence. It's clear now that in households where both parents need to work, childcare can raise problems. But there's helping and then there's being an unpaid on-call worker for very selfish and unappreciative employers. If you were being paid, would you be able to say that you had fair and considerate employers? Possibly not. No need to have big discussions or a falling out - you can only do what you are able and prepared to do at any agreed time. You've had your full-time children, so I was going to say 'not your circus, not your monkeys' now, but I can imagine that you don't find this situation in any way amusing. Try to back off and remember instead that your lovely Grandchildren have their own Parents....that's not you. Make your time with them count but also think about your own life, your own retirement, your own plans.
No need to even discuss or explain your 'odd ailments' - just start doing less and stop feeling unappreciated.