To start leave right now?

[Badbadtromance]

DM is elderly widow. I am sp and depend on her a lot for help. The problem is she can be very toxic and just plain nasty. Today she wanted to go shopping but my health means I can't drive too far. Although I had agreed to take her last night I just don't feel safe doing the drive. She has gone nuts. Screaming, shouting and insulting me. She is now sulking and refuses to speak to me. I know I let her down but its the throwing of all th help she gives me back in my face I hate. My siblings live away so do nothing. Its always me who does everything and it's very draining. I even had to turn down work because she had a hospital appointment she refused to re book. I am exhausted and fed up. No one understands just how draining it is. I feel like I have an extra child. I don't know how much more I can take.
She won't have any outside help, but the work load is such that I had a breakdown earlier this year. If she was nice I would happily carry on but its the spite I can't take but then when she is nice I think aibu or not. At her house now but really just want to walk out and go home with my kids. No one understands as she is oh so v nice to others. Its only when you get to know her. She has no friends and hates her neighbours so I can't just refuse her help can I? She can't even use the internet

You need to look after your own health here; you'll make yourself really ill. If she won't accept help from anywhere else, then to be completely honest; tough shit for her.
Easier said than done, I know.

What help does she give you OP?

You can refuse her help. If she can manage to be nice to other people then she is actively choosing to be nasty to you! You absolutely do not have to tolerate it. Get up and walk out and tell her you will continue to do this every time she behaves like this.

Don't rearrange your life to accommodate her anymore. Call your siblings and tell them you are no longer the default option and they need to pull their weight or it will be left to social services.

Your mother is bullying you and you owe it to yourself and your kids to get out from under her thumb.

She does odd school run or babysits or pays for kids clubs. Stuff like that. She thinks I'm selfish and ungrateful

You are doing way more for her than she is doing for you. It's not a competition, but she is certainly out of order to not acknowledge how much you do for her and to speak to you so badly.
My advice is to take nothing from her - not her help and not her abuse! Distance yourself and reclaim your own life. Being old doesn't entitle her to anything from you when she cannot he civil.

Step away, seriously. It will be the best thing you ever do. Otherwise you will end up breaking down (trust me).

She has no friends and hates her neighbours so I can't just refuse her help can I? She can't even use the internet

^
Yes you can. Give yourself permission to refuse. Do it now because she is a BULLY. It sounds harsh but as PP's have said, you need to for the good of your health and for your children as it can't be good for them to be in such an environment either.
Sulking is a form of control.
Refusing to speak is a form of control.
And she WILL have outside help if your not there - she will have no alternative. She is guilt tripping you.
Go home, make yourself a cuppa and put your feet up .Then when you feel able to, phone your siblings and tell them enough is enough, it's their turn. Don't explain yourself, just repeat enough is enough ad infinitum.

Stand up and take back your life my lovely.

I'm guessing then that she thinks you are selfish and ungrateful because you won't be at her beck and call. The fact she has no friends and hates her neighbours proves what a an arsehole difficult character she is.