Hi! I just started Year 13 and had a similar feeling when I started Year 12.
Are you able to get access arrangements/do you already have them? I struggle with motivation too, I was able to get 25% extra time, it helped me so much.
A-Levels are typically harder than degrees, but also, your Law teacher might be pushing too hard, if you're able to I'd try to ask around. 5 Pages in 30 minutes seems like so much I don't blame you for getting stressed over that, definitely check that. That being said I don't take A-Level Law so I'm not sure if I'm able to help you much on that one (maybe switch Law out if you feel it's not suited for you?).
With independent study I find it's easier if you stay in school or if you program yourself to have a designated work space, for example only use one corner of your room for work. It sounds dumb but I've been doing this for a while and it's got to the point were if I'm that designated work space I just start thinking of work or doing work absent minded. I also find white noise/documentaries/shows etc just playing in the background while I'm working helps. Use most of your frees as well, that takes a chunk of time off of the 15 hours.
As for public transport, just practice. Get on buses/trains, get your parents/siblings/friends to travel to places you want to go, eg. If you have a day out planned ask them if they'd be willing to use public transport, if not try taking it alone. I used to really struggle with the idea of going on public transport but after a few times it got so much easier, bring earphones with you if you don't bring a pair with you already.
I can't tell you what option to pick, A-Levels just aren't for some people, if I were you I would honestly stick it out a bit longer and try to adjust, if you feel like you can't adjust and it's too much after a while then I would consider other options. I find it gets better once you have adjusted and routined yourself.