is it possible to lose velocity from throwing too many curveballs? im 15 years old 5 foot 7 and 140 pounds

Let’s word it this way: to develop a better fastball, you’ve got to throw it.

Actually, to develop a better anything you have to throw it. Fast ball, curve, whatever the pitch, you have to work at it until you get what you want with it. I remember when I learned the slider—I got the hang of it in about ten minutes, but I knew that this isn’t a pitch one can get overnight, and so I worked at it; it took me a little over eight months before I felt comfortable enough with it to use it in a game. And it became my strikeout pitch. 8)