There must be some societies or sports clubs that your interested in that you could join at uni, sports clubs in particular usually have very large socials where you could meet loads of people.
Unfortunately you are right in thinking most people have their circles of friends now but that doesn't mean you can't get into these circles, it just means you now have to do more of the approaching.
Unfortunately I agree with "cole-slaw" statement but if you are as social as you claim to be, then meeting people on student nights out or in the library should be sufficient, you don't have to drink to get involved but I agree drinking is a huge part of socials, at the end of the day having a great uni experience depends on how fully involved you get, if I was you I would go and get involved with as many things as possible when you go back.