First of all, give yourself permission to back out and leave.

I tend to think catastrophically. There are some great Decatastrophizing Worksheets on the internet. These worksheets ate helpful in that they ultimately lead you to formulating a plan should the case be the situation gets difficult. Also ask yourself these question making a mental list: how is avoiding the gathering helpful to you? how is attending going to be beneficial. I know, easier said than done.

(Gee, I really need to remember doing this for myself.)