I struggled with the same thing when I was in cheerleading and never got over it. Now that I no longer do cheerleading, I know that it is the same as any other thing that makes you nervous. Remember when doing a cartwheel made you nervous, but seemed like the coolest thing ever when you finally nailed it? Try to remember that feeling. Just calm your thoughts and think of yourself doing it perfectly, eventually, you will get it:)
If you have a trampoline practice on tht! :) Tht helped me get mine. Once u feel confident doing it on the trampoline then try it with out jumping......if ur trying to get a round-off on the trampoline but its not big enouh then do a handstand snap down back hand spring (with out jumping)!! It helps alot!! It helped mee!! :D
Start off slow with drills and always make sure you have a supervisor just to make sure you don't get hurt once you feel comfortable doing it on a trampoline, try doing it on the ground with a spot. As you get better tell the spotter to move away each time you do it. Eventually you will become confident enough that it won't freak you out but at first all new tricks are scary.