You don't attempt A level Maths if you got an E at GSCE. Ok so you didn't revise/try...still...don't attempt it unless you are breezing an A. Been there done that having got a C (should have been a B) and had to quit it was too hard. You just have to accept sometimes you can't "get" maths quick enough.
As for going back to do them all again. I'd also advise not to...at all. Plenty of people work up from nothing with no grades. Al about attitude and adapting/learning something. Nothing I did at school helped me with my career and I even did A levels and started a degree which I quit to go work in IT. I never studied anything to do with IT and I gradually just worked my way up. I am on better pay than a lot of people I knew who went to uni. Majority of them are in so so jobs too, it just took them even longer to get there. Some are complete bums with no jobs still. Some habe poor jobs/careers.
Just saying that imo at 20 years old, for me, you would be better off not wasting a year or two even three more with more education. Just get out into work.