If you think you can’t do it and you don’t want to go back to day 1, just don’t go. There will be more parties.
If you do go for whatever reason, have a plan. Decide what you will drink. How you will deal with anyone offering you a drink of asking why you’re not drinking - which is not at all a big deal but it’s best not to let this catch you off guard. Have fun anyway! Be silly, dance, speak to people and savour the fact you will remember it all. Leave early, especially if everyone is getting wasted - it gets boring and they won’t remember. You may be surprised at how much less other people drink than you might have imagined. You will likely not be the only person leaving early.
The thing to remember is that no one else cares about your drinking (or not drinking) as much as you do!
There are a few links to more detailed conversations about sober socialising in the FAQs too 