I have that problem too. Whenever I get really excited about something, I feel anxious and start stressing or saying things I normally wouldn't say. I think it might have something to do with the ADHD, but everyone have their own reasons for anxiety.
My suggestion is that you pause for a moment, think about the situation, breathe, and tell yourself it'll be okay. What you're getting excited about is a good thing and you want to make said good things a positive memory. You don't want to look back on, say New Year's, and think about how you were holed up in the bathroom throwing water on your face to calm down. You want to remember sitting around the living room table with your family and friends, munching on delicious om noms, counting down until 2014.