Matilda is spot on, on this OP...
I do feel your pain. You are currently drowning in a whole barrel of "stuff to do" and "stuff you can't do much about." I would suggest that some of this is just stuff you've got to grind through step by step and accept that your M is never going to be the ideal parent - not a cheerleader but a very strident theatre critic. That is her problem and her loss, she's missing out on having a better relationship with you.
It's OK to have a lot of stuff going on... and it is absolutely OK to not be able to wave a magic wand and have everything going brilliantly all of a sudden. These are big life problems and you are doing your absolute best. Accept that and get snappy with anyone who wants to imply that because you have life issues to deal with and things are not magazine perfect that they have the right to criticise you, make you feel bad about yourself etc...
All free childcare has a price.. and you are currently paying. If that is worth it, carry on. If not see less of each other.
As Matlida suggested... Just stop telling her stuff. Stop.. put your hand over your mouth and pinch yourself if you ever feel your story bursting out. Stop handing her the bat with which to beat you. And while you are at it.. I'd have some phrases up my sleeve to close down certain discussions. I'd also have some statements like, "That's not important right now - Is it raining?" you know the sort of thing.
She's unlikely now to change. but you could change the way you react to her. If you stick to very neutral topics, could you get along a bit better? It's better to make decisions in advance.. ie don't ask what do you think we should have for dinner? etc...or sound indecisive. Just have arrangements planned in your own mind. Then there's no need for discussion or critiques to creep in. And you are more in control then.
When she's starting on her antics, try to not to give her views so much importance. She grew up in another era, very different to your own. She doesn't seem, from what you've said, to have the capacity for sympathy or empathy (she probably wasn't shown this by her own parents) .
So Find someone else to confide in. But also, tell yourself that you are doing well coping under an avalanche of difficult circumstances. Most people would flounder from what you currently have on your plate. So getting through day by day is actually pretty good and you will resolve each issue in time, which is also pretty good if you think about it! If you have confidence in yourself, some of these comments will sting less and be apparent for the nonsense they are.
To quote Matilda "Ask me how I know."