Hey,
My heart goes out to you on this. It sounds like your living situation is causing you a great deal of anxiety and regret, which is really hard to shake. I've had similar feelings before (actually when I did my year abroad) and in all honesty I know how consuming the worry can be when it feels like you've made the wrong decision.
The short answer is yes, these feelings are temporary and will go away, especially those you have regarding starting studying and getting the trains in. It's completely normal to feel like that at the beginning of a new year and in a couple of weeks you'll feel much more settled. You won't be the only one who hasn't done much work over summer and I'm really sure you won't be behind.
It sounds like you have some regret around deciding to live at home. What I'd say to this is wait until you're a few weeks into term before you let this regret develop too much - you haven't even started the year yet! If you still feel like you've made the wrong decision, try not to let it consume you. Turn this regret into excitement to move out and use it to fuel your anticipation for your year term/year abroad! You've still got literally years of your student experience ahead of you, so you haven't missed out on your chance at student life at all.
In the short term there isn't much you can do to change your living situation, so try some strategies to cope with your anxious feelings. I find journaling helps - it sounds like you've got lots of really valid and thoughtful reasons why you made these decisions, and writing them down may reiterate to you why this was the best decision for you right now. Also just trying to do something distracting can help break that chain of negative thoughts, like watching Netflix or exercising (I hate exercising but unfortunately it helps me so much).
Take it from someone who's been through similar - it will get better, it might just take a couple of weeks. Try not to let the regret consume you - it will most likely be a really great second year, and if not, take comfort in the fact that it's not forever.