Not sure what career I should go into? Computer Science/Accounting/Business e.t.c

I'm going to continue with sixth form and then leave for Uni.

You are young, you've had a setback in your career but you have plenty of time. Your best options are either study maths, independently with a tutor and sit the exam as a private candidate if necessary, or concentrate on 3 A levels for now, apply to courses that don't require maths and see if you get in either through UCAS or clearing. If you don't get a place take a gap year, study maths then when you can devote more effort to it and reapply. If you are studying only maths and work hard you should be able to achieve at least a C grade, possibly more than that.

You wouldn't find a 6th form college that would just let you study one course but if you get 3 A* grades you'd find somewhere that would possibly let you do maths AS and A2 courses and carry on the Psychology to A2 or you could just do AS maths and something that would allow the college to get its funding for you (like photography). Or get a job and pay for a tutorial college to teach you maths. You might want to see if you can get your maths papers back to see where you went wrong in the exams, I'm assuming all papers were poor and there's no chance a mismark on one paper pulled you down.

You don't need to do 4 A levels for university, 3 and an AS are fine so I don't know why your school are pushing 4 at you.

I know, i'm suggesting that if you don't end up with maths (for whatever reason) you should consider applying to degrees with foundation years.