I've been a rubbish mum/gran

To be honest, she sounds quite brutal, and as a result you have had your confidence and self esteem badly knocked. If you looked after your grandson two days a week and also for four days after the second baby was born then you have certainly done your share of childcare. She should be grateful! Especially as you have health issues. I do not think you have let her down as you seem to think. What has happened is that she has made you feel as though you have let her down.

During that period after the birth when she says she wanted your help, where was her husband? Surely the very reason he was off work was to help her.

I realise there are two sides to every story, but if she is ignoring your health issues and bad mouthing you to the rest of the family then that is appalling behaviour. Those are two things which should never happen. She needs to understand that you have feelings too and that your health really is an issue, but only time will teach her that. Can you get one family member on your side to speak up for you?

I'm not sure there is anything you can actively do, as she doesn't seem to have any empathy for you. Is she your only child? If so then maybe step back for a while until she realises she still needs your help. Keep the lines of communication open, but don't chase her. You will come across as needy and you want her to need you. Send texts to say you hope all is well etc. and birthday cards. When you send a card just write in it that you love her and to let you know if she needs anything. In fact a pretty card sent in the post saying just that might tug at her heart strings.

Please dont cry. You have not let her down and she did not ask for help. Over on mumsnet they complain that MIL especially is at the house too much.
I honestly think your son and dil are nasty for taking sides. It's not of their business.
I would try once more to explain to your daughter then let it be. I do think she is a bit entitled especially as she knows of your health conditions.

It's about time your daughter showed sone empathy.
Parents don't have to become martys when their ADULT children give birth.

I had a similar issue , though slightly different circumstances , with my youngest son and his wife.
When my grandson was born 12 years ago, I was roughly your age, going through a very distressing and difficult divorce, working part-time, doing school runs for my daughter’s children etc etc. My son asked me if I would look after the baby at least one day a week, because my d.i.l. wanted to return to work. I just could not help them. I only had one free day for catching up with my chores and I was not willing to give that up.
So, I sat down with them and explained it all. I offered to do baby-sitting, if they wanted to go out for the evening. I don’t think they were too happy, but that was all I could offer. I also felt that they were financially well-off and could afford to employ a child-minder or whatever..
To cut a long story short, they understood,
My d.I.l. did not return to work. Our relationship has not been affected. I babysit when they go out for a meal,
I have retired now and everything is hunky-dory. Just last week I stayed overnight with the boys (they have two now), to allow them to go out for a meal with friends .
You have to be straight with them. Your health is your priority.
I am shocked that the rest of the family are not sympathetic to your situation.
I hope things improve