Hi OP,
So on the one hand you want to leave but feel trapped financially, but on the other you want to stick together and try and make it work. Perhaps you need to set the whole issue of money to one side for now and just look at the relationship by itself - can it be saved? do you want to save it? is it worth trying to save (it takes two)?
If you do really think the relationship is over, then you can leave. I hear what you are saying about wanting to provide financially, which you certainly can and will do, but just not for a few years yet. Take the financial help from 'the system' that you need now, because you will be paying taxes for decades once the children are older.
You will have to 'share your children' - they are also his children. You are presumably sharing them now whist in the same home as him.
It is a false comparison to compare your mortgage to renting, because the benefits system won't pay for a mortgage but it will (mostly) pay your rent. You won't be paying the rent, so it doesn't matter how much it costs.
There is no need to go back to your mum's if you can scrape together or borrow the deposit and rent in advance on a rented place - you may find that if you go to your mum's the benefit system won't help you leave there.
However, given that you are in two minds, maybe wait a year and try to give the relationship another chance - have a really good talk with him and maybe try counselling - ask your GP if anyone can do it free or cheap, as it can be expensive, but it would be worth it if it saved your marriage. Only you know if there is honestly any chance of it being saved.