The first thing you should do, in my considered opinion, is make sure you’re not telegraphing your pitches. I’ve been thinking about this, and I recall several situations in the major leagues where a pitcher was getting belted from here to Timbuktu because, all unawares, he was tipping off his pitches. Whitey Ford, for example—he had just come up to the Yankees in 1950, and he had started a game, and the opposition was just eating him up until a teammate called his attention to the fast that the first-base coach was calling every pitch. The next day he went to the bullpen with pitching coach Jim Turner and teammate Ed Lopat and threw from the stretch for some minutes, and Lopat spotted the problem immediately: Ford was positioning his glove hand one way for a fast ball and another way for a curve, and because he was a lefthander it was no trouble for the opposition first-base coach to pick up on it and relay it to the hitters! Lopat told Ford what he was doing wrong, and the problem was corrected in a bullpen session. So it behooves you to make sure you’re concealing your grip and not doing anything that could give away what you’re going to throw next.
Next, make sure you’re throwing everything—and I mean EVERYTHING—with the same arm motion and the same arm speed. This is another way a pitcher might inadvertently telegraph—slowing down his arm speed, for example, or make some little movement on the mound that could tell the batter what to look for next. You say that the opposing hitters have been zeroing in on both your fast ball and your curve? I say, look into it, because you might well be tipping off those pitches without realizing it. Also, you should conceal your grip until the last possible moment.
I know, most of the other respondents say don’t think about acquiring another pitch just yet, but I’m not so sure. You might consider another variety of changeup. My old pitching coach, who was a member of the Yankees’ Big Three rotation, once told me that just about any pitch could be turned into a nice changeup, and he demonstrated a few such for me. You might think about something like a palm ball, which is easy to pick up and easy to throw and which is thrown with the same motion as for a fast ball—or a circle change—or (a favorite of mine) a knuckle-curve, which can be a devastating pitch when you get the hang of it.
I for one was a snake-jazz pitcher, not much on speed but with a very good arsenal of breaking stuff to which I kept adding, and you know what my strikeout pitch was? A slider which I nicknamed “Filthy McNasty”, after a character in a W.C. Fields movie because that was exactly what it was; it had a sharp late break to it. I threw sidearm exclusively, and I used the crossfire extensively, and I got the batters out with it because they couldn’t time it.
In addition, my pitching coach let me in on a little secret—an aspect of strategic pitching. Instead of putting more stuff on the ball, he would take some off it, and that was how he would deal with power hitters—at least the ones who thought they were. Case in point: a guy named Walt Dropo, who played for Detroit and Boston. He was a power hitter—or at least he thought he was. He would come to the plate drooling and licking his chops in gleeful anticipation of the delicious goodies awaiting him. And the pitcher would take even more off his stuff. Result? A big fat strikeout, or a weak dribbler to first. And as the batter would return to his dugout foaming at the mouth and muttering all kinds of unprintable imprecations, the pitcher would yell at him "Dropo, you’re just a lousy hitter!"
These are just a few things I have in mind that you might try out and see if you can mess up the hitters’ timing—and their thinking. 8)