Not sure how to help you get the nerve to jump something the first time, but...
...once you have jumped something, I suggest that you jump it *every* lap. Some people will wait until they ar halfway to a jump to decide each lap. If you have those doubts, you'll always be hesitant and more likely to have problems.
I still back off and don't jump some things if I feel my approach is wrong, but I enter every turn planning on making the following jump (if it's something I have jumped before).
As far as personally, I watch someone else hit a jump to note their body position, speed, gear, line, and how hard they were on the gas. I've had people follow me over jumps to get their timing down, but that doesn't work for me.