I was a snake-jazz pitcher long ago. I didn't have a fast ball to speak of, so I had to go to the breaking stuff right away, starting with the curve ball which came attached to my natural sidearm delivery. I picked up a few other pitches---the palm ball, the knuckle-curve and a bit later on---THE SLIDER, which I nicknamed "Filthy McNasty" after a character in a W.C. Fields movie because that was exactly what it was---my strikeout pitch. I had advice and instruction from one of the Yankee pitchers in the early-to-mid-50s---what an incredible pitching coach he was!---and one of the things he told me was that just about any pitch I had could be turned into a nice changeup. I won a lot of games that way.
As far as people telling you not to throw the fast ball one way or the other, my advice is "don't listen to them". They are actually two different pitches. The two-seam fast ball is also known in some quarters as a sinker, and I've seen it thrown in major league games, and it is one of the deadliest pitches in existence. The four-seamer---well, watch Mariano Rivera, who will throw that one sometimes, and because it tends to go up in the strike zone you'll find the batters hitting under that one and popping it up. So, you do whatever is comfortable for you and gets you better control. :idea: 8)