I dreaded my first heavy drinking party after I stopped drinking. It was a "bar night" with a lot of the people that I did heavy drinking with previously (and I had to go because it was to celebrate the life of one of them who had died).
But, what I found was that if I could get through the first hour of the party, it got easier and easier. So, if you are struggling, just focus on not drinking for the next 15 minutes - and then the next 15 - and so on. It actually gets pretty funny to see how silly the drunk people get. Keep your glass filled with soda and enjoy the conversations.
And surprisingly, I had a great time at the end (and no hang-over in the morning).