Like Zita said pretty much, the fastball is like any other pitch, you may be used to just grabbing the ball and which way in the field but you have to get comfortable with your fastball grip off the mound. I would be extremely surprised if your mechanics were perfect as well. I’ve been pitching for years and I know that I can still improve, if you were able to just pick up pitching mechanics completely fine like you said, that would be a major accomplishment. My guess is there are some very serious parts to your motion that you have overlooked. Also, pitching requires you to develop two motions, the wind up and the stretch, of course you could just use the stretch but most people prefer to have a wind up unless they are a relief pitcher. You have to get comfortable out of both positions, not just one. There are many young pitchers (I say young as a relative term to refer to someone who is new to pitching, you are only a year younger than I am) who only work out of the wind up when they practice and struggle mightily once they get runners on. Not only do you have to be comfortable from the stretch but you have to be quick enough to the plate to give your catcher a chance to throw the runners out to, otherwise those walks that you give up will turn into easy doubles for the other team as they can just swipe the bag. I had a lot of trouble with runners early on in my high school career but I developed a good pick off move to keep the runners close to the bag and a slide step to give my catcher a chance. You even have to be careful to be quick enough if there is a runner only on second as they can and WILL time you and as soon as that leg lifts, they are off to the races. Another important part of keeping runners honest is to mix up your timing, if you feel like you have been doing the same thing over and over and the runner is timing you, just come set and hold it until either the batter says time, or the runner takes off. Yes, sometimes the runner will take off because they have been timing you, it has happened to me many times when I used this method. I realize that this was a giant post for you to read but I just wanted to make you aware of some of the things that aren’t so obvious when you start pitching but are extremely important.
As far as my personal tip besides getting comfortable with your motion and grip, keep your shoulder closed, throw through your shoulder, and throw through the catchers mitt, not to the mitt. Also, consistency is the most important part, repeat the same motion every time, but make sure you are comfortable with it.
I apologize for the wall which may have caused bleeding of the eyes upon sight.
