Melavai, it's called exposure therapy, or desensitization. Look up specific phobia disorder.

I still get nervous when 1) I am riding in an unfamiliar area and I don't know what is coming and 2) downhills. I am better than I used to be, and I just force myself to go. Sometimes that means I am the last person down a hill, but at least I know I'm not going to be the last person up the other side .
Once I decide to do something, I do it.