Stepson doesn't live with them.
From OP's OP:-
If DP wanted him here full time I'd have him.in an.instant.
Rev, you're right, he is 'unrealistic'. Some might say selfish, lazy, greedy:
He doesn't work and hasn't properly in two years due to one day of college a week.
we now rely solely on my ESA and tax credits as he quit his job in a kitchen that lasted a month as I was getting so bad in the nights.
He's choosing to live off YOUR sick benefit.
He's eligible for JSA but has told them he won't quit his course which means he voluntarily doesn't meet their criteria.
The jobcentre told me this once. They were wrong. Did he actually pursue this? Check with the CAB, or even the DWP websites? Post on any benefits forums? Check with his course provider? That would have been the obvious first thing.
After the nextyear of his electrical course he wants to go to university fir three years full time. He doesn't seem to care that he has three children to support.
No, he doesn't seem to care.
Look, Rev, where you see a lovely, missing DP, everybody else sees a selfish sponger who's not above squeezing the last few quid out of a seriously ill, single mother.
If he were genuinely interested in supporting you he would have applied for benefits to help out - carer's allowance, maybe, and would in any case have tried harder to get JSA. Or work part-time; even if he'd only brought home £80 a week it would help, yes?
Fibromyalgia isn't something you can just get over. It requires a lot of rest and minimum stress. This man adds to your stress. If he can't support you, he is NOT an enhancement to your life; he's adding to your problems.
I hope he doesn't come back. Not without a job and an attitude adjustment. 
I hope you can find enough friends, neighbours, charity workers, social carers and medical staff to lean on while you feel your 'loss' and start taking your recovery seriously. I hope you'll be able to feel glad of the good times you had together, while also being glad to lose a millstone from around your neck. And I wish you well.