I had the same problem, until one day i was just in the mood and dropped off all sorts of shit. Definitely the best thing to do is start small and work your way up, if you've got a wall that gets bigger gradually use that and spend a lot of time there, and get nice and comfortable and don't let anyone rush you. Always look at the drop from the bottom, as drops look tiny until you get up there. So always have an image in your head of what the drop looks like from the floor, rather than focusing on what it looks like as your doing it. Also, what helped me was 'try' and do a drop 2 ft bigger than the one you're actually trying to do. Don't actually do it, but get on your back wheel and turn as if you were, and then back out. When you go back to the drop you were originally going to do, it looks a lot smaller, and you're more confident on your back wheel as you've just done it all on a much bigger wall. Make sure your technique is good, if you're not fully confident on your back wheel, don't do big drops and practice on your back wheel first. Same if you don't drop properly. I've seen people just backhop off walls rather than get right on the edge and lower their ass and from wheel and it always ends blackly. A good technique can get rid of 80% of the impact you would get with poor / no technique. What kind of heights are you dropping?