Stopping drinking, for life, isn't easy. Most people fail. I, and many others here have been doing it successfully for decades now however, and while I used to wonder why, certain things are becoming really apparent to me.
When I stopped drinking, it was a matter of life and death. And I respected and treated it as such. In my first year of sobriety, if something was even the slightest bit of a threat, I stayed away. I missed parties, I walked out of places without saying goodbye to anyone if I felt uncomfortable, and I didn't give much thought at all to what I "might have been" missing. It was the party, or my life. Being cautious and staying away from people, places, and things that might make me want to drink was incredibly important. In the beginning.
Because of the importance I put on all that in early sobriety, I was able to build a solid foundation. In retrospect it was a tiny investment that yielded unfathomably huge returns. And it didn't mean that quitting drinking meant my life was over. That I'd never have any fun again. It meant the complete opposite. Because of those tiny little sacrifices I made in the beginning I feel I was granted a life of complete freedom from alcohol. I go anywhere I want now without concern over my picking up a drink. And it's pretty much been that way since after my first year or 2 sober.
How badly do you want to remain sober? How important is this party? How important will this party seem 5 years from now? Is not drinking a matter of life or death for you? It might not be at this point, but if you really want to be successful at sobriety, it might be a good idea to treat it as though it were. For me it was clear that it was. Stuff like this was a no brainer for me. My sobriety was my #1 priority, and still is, though I don't have to treat it in the same way as I did in the beginning.