Try jumping up to big ledges first, then you'll realize it's all in your head. I need to go for it more too, I have trouble with higher rails.

I have eaten some serious shit in the course of my soaping career. I overshot a rail and slid down a hill covered in stupid bushes once. A hunk of branch stabbed into the palm of my hand. I banana-peeled super bad on a taller rail and racked my gooch severely in the same session. It was a bad day for soaping. I pulled the whole rail later that month though, it was one of my first real "vertical" grinds. Later my buddy Phil broke his wrist soaping that rail. We sessioned it quite a bit, because it was just up the block from my house at the time. I tripped on the lock-in jumping up to a backside on a tall skinny concrete ledge, stepped over my front foot and went face first into the asphalt parking lot on the other side. I rolled out of it but jacked up my shoulder pretty bad. Put me out for a few weeks.

I would suggest wearing protection to have more confidence and using maybe a chair or something that you can step onto before grinding. That way you don't have to jump very high and can get used to higher rails.