Great advice from username299
When I read your op, I had two opposing thoughts.
The first is that I agree with you in so many ways about society not being fair on working mothers. Childcare costs are horrendous and there aren’t enough subsidised holiday clubs. Also you have come through a lot with an ill baby.
The other thought is that you are catastrophising a lot op, As pp said, you are volunteering and studying and have been in a caring , as well as a parenting role for three dc, so a job might be easier than balancing all of those things, especially if you find one in the same area as your volunteering position , which brings you fulfilment.
I think it would definitely be a good idea to finish your studies . And maybe find a p/t role to build up your confidence at the beginning. You don’t have to rush and do everything all at once. Step by step is good enough.
As for your spending, and lack of saving, you do need some advice and help with that but I think it would help considerably if you had an honest talk with your dh. Tell him about your debt and put a plan in place. Share your worries with him. Come up with a plan together.
As pp said, he was somewhat involved in your pregnancies too!
Did you ever talk to your dh properly about joint income when you had to take redundancy, and saving your payout, or what would happen when it ran out?
He’s benefited from moving in to your property presumably?
And op, speaking frankly, just put the sil out of your mind. It’s tough but you are not in the same position as her and you need to be focused on your own path. And few mothers of three can be sahms nowadays. Also, trust me, being a sahm doesn’t necessarily guarantee that you will be the greatest parent in the world either. .
You have between now and May to really focus on getting that pt job. Ask around for potential opportunities where you volunteer to begin with.
You can do this op but breathe, write an action plan, and start tackling all of the issues gradually with more input from your dh.
Good luck.