I feel like that sometimes, as I have been in a similar situation as you. I didn't care so much about my grades as I do now, and so I messed up my GCSEs as well, and spent most of weekends with the wrong crowd. Now I spend more time doing work, and luckily, still have a lot of NICE friends, although I don't go out as much as I'd like to.
I think you need to try and find a balance between socializing and work. Right now, work on your social life. Aim to go out with friends at least once or twice a week. There's nothing wrong with your confidence..it's still in you somewhere, waiting to come out. And you CAN make friends...you had loads before.
So when you're at the party, remember how you felt before in your more confident days and that you still have that confidence. Be friendly, smile a lot, and think about the situation there and then. Not about what you look like, or your height. There will be people there who are all shapes and sizes. Have a few drinks, and relax. It will be fine.