Perhaps when you were invited, you felt good to be thought of, to be included, to receive an invitation that says "We want you." That does sound great.

Now as the reality of the event approaches, you are feeling frightened about being there. Something about being there doesn't feel so good. What are your thoughts at the time you feel not good about it now? Is it that it is unknown? That something unknown might be expected of you? That you will be judged?
Sometimes if I can identify the specific thing (or things) that are triggering me, then I can think about them and see if my fears are reasonable or if I am scaring myself with my thoughts.