What? Nobody ever told you about THE SECRET??? Well, let me be the first.
THE SECRET, which I learned many moons ago from watching how three Yankee pitchers did it, involves getting your whole body into the act of pitching. I used to go to the old Yankee Stadium every chance I got, and I watched those guys, and I saw what they were doing and how they were doing it. They were all driving off the lower half of the body, using the legs, the hips and the torso in one continuous (and seamless) motion, creating a nonstop flow of energy all the way up through the shoulders and arms to the fingertips and thus generating more power behind their pitches---thus, not only throwing harder and faster (even Ed Lopat who was not a fireballer) but also taking a lot of pressure off said shoulders and arms so that those extremities seemed to be just going along for the ride. I saw this, and I made a note of it and started working on it on my own, and as I practiced this essential element of good mechanics I found I was doing the same thing they were. Even though I was no fireballer I was throwing harder and faster with less effort---and nary a sore arm or a sore elbow or a sore shoulder or a sore anything else!
The best way to get started on this is with something called the "Hershiser" drill---you can look it up on this website or the NPA for instructions on how to perform it. There's no special equipment needed, just a fence or a wall, and the whole aim of this drill is to get your hips fully involved in the process. You see, the hips are the center of it all, the connection between the lower and upper halves of the body, and this is something a lot of batters don't know---the lower half is the real key to a pitcher's power. Once this connection is established, a lot of other things will fall into place. One thing---you might want to get together with a good pitching coach, preferably high minors or even major league, to work with you on this and a few other things---and your "general frustration" will cease to exist. That was what I did for several things; the aforementioned Mr. Lopat, who besides being one of the Yankees' aces doubled as an extra pitching coach for the team, helped me refine them and expanded my repertoire. One more thing---I don't know what your arm slot is, but if by any chance you can throw sidearm you might do well to pick up the crossfire---it will give you extra pitches with less effort.
And there it is. Best of luck, and greetings from the Goddess of the Slider. (smile)