I think you need to take an hour to yourself before the event and meditate, focusing on your breath and also visualising how you would like to be in the bedroom.

Also make sure you are well lubricated so that any tension does not translate into your vagina. If you are tense, perhaps you could ask your partner to massage you. He will see this as foreplay, but it will also have a practical use.

I find it most helpful in social situations to focus on the other person, rather than my own feelings. Open up with curiosity about your lover and focus your attention and affection on them and you will be too busy to worry about yourself.

Good luck