Lara,
I'm so sorry to hear that. I used to have the exact same problem myself.
Well, there are no quick solutions to what you are experiencing, sorry to say.
Performance anxiety, as it is called, is a common problem that a lot of people struggle with. I did for years. It wasn't until I learned what anxiety is about, how it affects a person and how to react to it that I overcame it myself.
The good news is you will overcome this and your public performances will improve tremendously!
In your case keep it simple from now until Christmas. Don't think you have to change your playing, just a few little things. So, one of the first things you can do is to visualize yourself playing as wonderfully as you know you can when the time comes on Christmas Eve. When you have a free moment in your day visualize youself in front of Church playing like Gabrielle!
If you can, practice in the Church so you get comfortable being in the spot you will be playing. If you don't usually play at that time of the night, try it out this weekend.
On the performance day any thoughts of missed notes or thinking 'What if..." should be put out of your mind and replaced by postive thoughts, such as "I know I am excited and I am ready to play my best tonight" or "I wouldn't be up here playing if my Church family didn't believe in me" or as simple as "God is on my side and I can do this, there's nothing to worry about". Also, practice deep, deep breathing. In through the nose and out through your mouth like you are blowing out a candle. Concentrating on deep breathing is one simple way to relax the nerves.
OK, these are some examples, and since I assume you are comfortable doing so, don't forget to ask God to settle your nerves, after all He wants you to succeed and He has a soft spot in His heart for nervous trumpet players on Christmas Eve - trust me on that!
Now, after Christmas, tell me how it went and I'll have some resources for you to look into. These are simple things, nothing dramatic, I just don't want you to have a bunch of new things on your mind right now.
Greg