I can give you a couple of suggestions that might help. First of all, you need to disregard those coaches who think that pitching is as natural as brushing your teeth. It is not. I would try to find a coach who really knows his stuff---preferably an active pitcher or one who's not far removed from the game---and ask him to work with you on some of the things you feel you need to know. I was lucky when I was a kid---at the age of 16 I learned to throw the slider, and I learned from an active major-league pitcher who doubled as an extra pitching coach for one of the great teams. I worked with him for a little over three years, and what I learned from him was nothing short of priceless.
And here's something I used to do when I was a little snip of twelve. I had a natural sidearm delivery and a pretty good little curve ball, and I picked up a few other breaking pitches, and I used to work on things this way: I would find a good catcher, and we would mark off a pitcher's rubber and a home plate 60'6" apart, and we would play a game we called "ball and strike". The purpose of this was to sharpen up my control, and the catcher would position his mitt high, low, inside, outside, down the middle, you name it, and what I had to do was get the ball into the pocket of that mitt. It was more than just a drill---it was a good workout and a lot of fun besides. Even well into my playing days I would continue to do this; I didn't have a fast ball to speak of and I had to become a "snake jazz" pitcher, and some of the stuff I threw required a finer degree of control than others, so I continued with this little game of "ball and strike".
And you might think about what pitches would serve you best. Are you a fireballer, or a finesse pitcher, or a little of both? One thing I would definitely advise---stay away from the screwball. That's a pitch that if you throw it too much and too often will literally screw up your arm, no pun intended---look what happened to Carl Hubbell! I remember my pitching coach asking me about it, and when I told him I didn' t throw it, even though I knew how, he said "Good for you. You don't need it."
Above all---stay with it. You have the raw materials; you just need to refine them, and that's where a good pitching coach can help.