In the nicest possible way, you need to:
a) get out of working in schools
b) stop feeling sorry for the consequences of your own choices.
To explain these remarks, I too am an ex-teacher and there is a whole world of things to do out there that could potentially utilise your skills and experience. I was out of work for a while and it was fucking hard, I get that, but there really are other jobs out there - what about children's social care? SEND? Civil service? You don't have to stay hanging around in schools forever.
You are woefully underpaid as a TA - this is something that you could do with a L2 or L3 qualification, not as an experienced teacher. You don't work 'full time' - you are losing money due to the pro-rata nature of the role. Yes, you do a great job and the school probably love you - but they would, wouldn't they!
You chose to start a relationship with someone on a low wage. I am sure he's a lovely guy, but not having much money was the consequence of that choice.
You need to really think about the possibilities that are open to you rather than talking about giving your children more to your ex, just because he has more money? You are their mother, ffs. They need you in all kinds of ways and they also need you to be grabbing life by the collar and making the most of it.