I had the exact same problem. Here's how I overcame it.
First off, I tried to do it over a rail at about stomach heighth, so if you fall, you probably won't hurt yourself. I tried that for a while and got mastery of it. I then started on small ledges, and did the same thing.
Only about a week ago I went to the park and found three benches in a row and tried vaulting over them.
It took half an hour and a nasty scrape but I can do it now.
That might work for you, it might not.