Hi Michelle,
It happens to me all the time. I get really anxious doing things that i fear may cause a panic attack. Its just negative anticipation. It honestly happens to me EVERY DAY. Feel good when you get through it!
I've also tried positive self talk, but i've never been very good at talking myself down from anxiety. I just suck it up, and do whatever i had planned. Its not pleasant, but i hope that eventually the particular situation i'm facing will no longer cause anxiety, once i've proven i can face it several times.
Unfortunately, anticipation anxiety has been one of the hardest for me to overcome. Good luck, and let me know if you find anything good to help with it!
Beth