How will universities react if you do 4 years at sixth form?

You will be fine. You tried and changed after 2 years? If asked just emphasise how you made a tough decision, you made the right choice and it took a lot of determination. It is your grades that matter.

Made a lot of mistakes in my choice of choosing subjects which has brought my overall grade for other subjects and have been going through a lot of anxiety and a lot of stuff has come up which has not helped me. I was advised to take a gap year but im doing AQA chemistry and it was the last year for the old spec so teaching wouldn't be done by teachers for the old spec in this coming September. I know Im hardworking I've got a good grade in one A2 and A* in extended project, its just my two other subjects I've not been able to overachieve in this year. I really hope if I have to be an external student that universities still believe in me really...