There are many ways to throw a changeup, but they all have one thing in common---they're all slower than a fast ball. The trick is to throw with the same arm motion and the same arm speed as your fast ball while slowing up the ball---not as easy as it sounds, but it can be and has been done.
IT'S ALL IN THE GRIP. You have to find a grip that is comfortable for you, and there are several such---like the circle change. Assuming your hand is big enough, you form a circle with your thumb and index finger on one side of the ball and the other three fingers on top of the ball---and the key is not to grip the ball tightly. Another one is the palm ball---and I suspect you're gripping the ball so tightly that you're trying to squeeze the juice out of the ball, and that's probably why you're throwing it over the backstop. Don't. Just relax. And you might try more than one arm angle. I used to throw all my pitches sidearm, and I used the crossfire extensively, which made my pitching coach remark while he was working with me on the circle change: "I know you're going to crossfire it. You use that move with everything you throw."
He also told me that just about any pitch I threw could be turned into a changeup. Remember---the key to the whole thing is the grip, and you have to ease up on it. So you should experiment with different grips, find the ones that are comfortable for you, and work around with them until you get what you want---a nice changeup. 8)