If you wanna go to a respected uni later on in life then you'll have to go sixth form and go get the a levels. You just gotta keep going, working hard and you'll eventually get there. Also uni is a lot less stressful than sixth form, doing a levels is harder!
I'd say just keep yourself motivated and finish sixth form. As for taking an apprenticeship, if its right for you and you're sure you can't take anymore of sixth form go for it. But like i said i recommend to keep working hard and stay on.
I think you should go talk to your teachers and discuss your problems, maybe even get some help outside of school like a psychiatric or who ever u go to these dayss!