I have but not for 3/4 years. I've been told that I would make a decent Fly Half but I'm not sure.
You may be able to kick & have the ball skills to play fly-half, but there are other just as important aspects such as knowing exactly where to stand, knowing exactly what to do at certain stages during the match, knowing what moves would be most effective i.e utilising a support runner or having a wide move from 1st phase, knowing when to use your forwards and when to use your backs etc etc That all comes from experience not natural talent. You need 2 - 3 years, even at junior level, to properly understand the position. Your best bet, if you really want to play 10, is to start playing 12. I play 12 (and occasionally 10) and you are often drawn in as a first receiver during phase play, if the 10 is at the bottom of a ruck for example. So you pick up little bits here and there playing outside a fly half, you are right next to him, so you can see where he stands, hear him talking to his 9 & his support runners and after a season of playing 12 you could probably slot in at 10 and not look too stupid. Also join a touch competition if you can, you can pick up skills in touch that would help you in playing flyhalf.
Problem with Full-Back is that I am a bit hit and miss under the high ball.
Common problem, very easily resolved. It's technique and confidence. Have a look online for more tips, but the biggest mistake people make is, they don't watch the ball. They think they do, but they don't. What happens, is the kick goes up, their eyes follow it, then as it comes down they take their eyes off it. I don't know why, but a lot of people do this subconsciously. And just a split second with your eyes off the ball is enough to drop it. Follow the ball until in lands in your arms. Maybe look at the "Gilbert" on the ball and watch it the whole time. Also get your body in the right position, arms up in the air, pointing towards the ball and forming a cradle for it to land in. Get a mate to kick high balls to you and when you drop one, try and analyse why you dropped it and learn from that. This also applies for catching passes, people take their eyes off the ball, checking the oncoming tackler.