It may be, as you say, lack of practice. It may also be a question of mechanics—something might be out of whack there. And if other folks won’t let you throw hard, even with gear (catchers’ equipment, for example), you have to wonder—what are they afraid of? Getting plunked in the mask with a high hard one? What the blazes is the matter with them?
This last is probably the easiest to deal with. Catchers have that equipment for their protection, and most of the time it works just fine. You simply need to push that feeling aside. Now, about the mechanical aspect of it—you might want to have a good pitching coach watch you in action and have him tell you what you may be doing wrong, and then the two of you can work to correct that. Oh, I know—not everyone is an Ivan Rodriguez or a Jason Varitek or a Joe Mauer, but there are a lot of good catchers around, and you should make it a point to get hold of one of them and have them catch you.
The rest is practice. I remember when I was a little snip, 12 years old or so, and I would get hold of a good catcher and we would mark off a home plate and a pitcher’s rubber 60’6’’ apart, and we’d play a game we called “ball and strike”. The purpose was for me to sharpen up my control—the catcher would position his mitt high, low, inside, outside, wherever, and my job was to get that ball smack-dab in the pocket of the mitt (and what a satisfying feeling it was to hear that THWACK as the ball hit the pocket of the mitt!) These are just a few ideas I can present to you—hope they’ll help get you back on track. Good luck. 8)