After having to move to a completely new area of the country for my A Levels, and not wanting to, I was left in exactly the same position as you! I am now in year 13 and I am still experiencing the same thing. Luckily I did start to make two new friends in college towards the final few months of year 12 but then they left (to move to other colleges or start university), leaving me completely on my own this year. It is really hard because the same people sit at the same tables every day in large groups and not many people seem to mix at college.
Even though I am on my own most of the time in college, I feel relatively confident about making new friends in the future. In the summer after year 12 I went on a residential summer school and made friends very easily and I'm hoping it'll be similar when I start university. I still keep in very close contact with most of my old friends, but it is very difficult as they all go to my old school sixth form together, which felt really tough at the start of year 12 as I felt very left out. I have joined the Student Leadership Team at college which has really helped because although I've not made any friends that I sit with at lunch with or anything, I am friendly with all the other members and at least gives me people to talk to during some lunches.
The best advice I can give is just keep trying, join any extra-curricular activities you are interested in and mostly I've found in the last year and a half that you can become friends with someone when you least expect it! I agree the lonliness can be awful! Thank you for starting this thread as it is really helpful for everyone in this position to know they are not alone! I wish the absolute best of luck to you and anyone else on this thread in the same position.