Well stop stressing. Your best friend has asked you to be her bridesmaid AS YOU ARE, because she is your best friend and she loves you AS YOU ARE.
Stress is a massive trigger for comfort eating, so you need to stop flapping and think about what an amazing time you are going to have, what a good friend she is, and how you are going to do her proud.
Then stick to plan, and do your best between now and then. You will be fine.