<I've decided not to go. I'm having panicattacks about this and I know I'm not going to be able to force myself on the plane knowing that the seatbelts may not fit me. >
Sounds like you have a problem that's slightly more than just whether the belts are going to fit...
If you are for real, my sympathies.
Perhaps try here for some support.
http://www.pe2000.com/fearofflying.htm