Without seeing video of your physical makeup, your current pitching style, and so forth it’s kind of difficult to make suggestions. On the other hand, I’m not a mechanic’s coach, so in all honestly I’m going to suggest things that I’ve used with mature pitchers who have had other issues, but trying to get back in the swing of things.
I would suggest taking everything that’s right with your fastball and using that as a platform for most of the pitches that you’ll be experimenting with. The only exception, that I’ve found with this approach, is with a knuckleball. But then again, that’s me.
So, use your fastball posture to experiment with … oh… say an off-speed pitch. I say an off-speed pitch because you’re using all the body motion of the fastball, but the grip is the only thing that changes. So concentrate on throwing strikes with this grip. There are a ton of “how to” on the web to help you.
At first, I would suggest standing no more than 20 feet from your target - a catcher preferably… Get accustomed to the grip in your hand and how it leaves your hand. Take note of the ball in flight. Does it reach your intended target, or does it fall short, overshoot, etc. Make any adjustments with your grip only - don’t change anything else. Toss as many times as necessary, at about one third game speed. Get a feel for what your doing and what you’re trying to accomplish. As you gain confidence with locating your strike zone, kick up the game speed - but don’t move away from 20 feet. See if… oh… three quarters game speed gives you the same results as before. See if you still have control.
Now move 30 feet away and go through the same routine.
The learning curve is going to be difficult and test of your willpower, so don’t expect immediate results. Also, as your catcher, or the one catching your pitch, how he/she sees your pitch. That player should see a fastball posture and arm speed - but, the “pop” in the glove should be softer to the feel.
One of the difficulties with the off-speed at your age and at your level of play, is that it gets hit a lot. There are many reasons for this, but right now, I would concentrate on working with it. Besides, of all the pitches that work best- most of the time, is the off-speed. I would recommend going easy with the off-speed for the bottom of the batting order … 7,8 and 9.
Just be mindful that at your age, you off-speed pitch will have a little lobbing motion to it as it comes into the batter. However, as you get stronger, this will go away and you’ll have a decent incoming look-see by the batter that’ll add a difficult dimension for the batter to deal with.:
SaltySpit00n, trust your instincts with this pitch. When you’re spot on - it’s a career saver. When you’re spot off, just make sure there’s enough baseballs to last the game.