You need to keep remembering that physically and mentally you cannot do everything. You cannot make everyone happy.
Something has to give.
You have long term health conditions. The current set up could have a very negative effect on your health - what happens then? How will it impact you? Your three kids? In order to look after yourself and support them (and possibly your mother) you have to think of your health and NOT overdoing things. Sometimes it can be hard to realise we are burning ourselves out - there is not a good outcome to this scenario.
Your three kids are all at different ages but need you in different ways now. You cannot support them fully if you are running around after your mum.
Stop and think. Who do you need to prioritise. Your three children who rely on you, or your mum who could get others to help / pay for help / do more for herself.
Your mum sounds like she is doing a bit more for herself. Time now for her to do more or get more outside help. Tell her she is being selfish not using mobility aids - tell her that your children lose out out on time they need to spend with you as a result - tell her this is not fair, it’s wrong.
Don’t listen to her being negative for hours - tell her you can’t cope, you have to stay healthy and strong for your children, for her grandchildren and that she is putting too much pressure on you. If she cares, she will listen and act. If she doesn’t truly care, she will continue, so cut short visits and calls. You don’t have to listen to negativity.
Take care of yourself and your children who do truly need you.
This is written by someone who wants to support, who felt concern for you reading your post - and who has realised how very, very easy it is to reach burn out.