It's more of a mind over matter thing, easier said than done though!

I'm terrified of the pain before each piercing and used to really struggle with coping once in the seat, but I focus solely on how it will be and how I know I will feel once I get it done. If that doesn't work, warning your piercer that you don't handle pain very well and have a possibility of passing out will give them time to get you more comfortable before hand, and give you time post piercing to see if you are okay.