I'm at uni doing law
No mean feat! I didn't go to uni and I have never been out of work - I never expected to be out of work even without a degree. The fact that you ARE going to uni and doing law is a great thing and means you must be pretty intelligent. So getting a job should be within your remit too.
I'm finding the course hard
I imagine most of your fellow students do too. Law is a respected degree BECAUSE it's hard. Get any guidance you need from the uni. Keep at it.
I know I'll never get a job afterwards as I have autisum and dyspraxia and also mental health issues
Sounds like catastrophizing to me! How do you KNOW this, exactly?
How do you explain the people who have these issues and other challenges besides who all have jobs?
I'd never cope with a full time job and I can't imagine I'm a suitable candidate for a part time shop job
Again, you seem to think you have the gift of prophecy. Where on earth are you getting these ideas from?
Your problem sounds to me like (1) lack of confidence - look at getting some CBT or whatever chimes with you (2) forget about the future for now - invest your energies in your law degree (3) get biographies and movies about real-life people who've overcome all sorts of odds to make it.
You are young and smart. Don't throw away your opportunities on fear of the future. Just keep taking the next step and don't look too far ahead. No-one can predict the future - not even law degree students 
I wish you the very best of luck!