There won’t be many kids that can hit your fastball. I’d stick with that primarily. You must throw some sort of off speed pitch as well. You don’t need to use it often, just enough to keep the hitter off balance. Whatever off speed pitch you have that you can throw most accurately should be what you use.
The problem with having a dominant fastball can come from a couple of areas. You get fewer swings because you are overpowering the hitters. Also, umpires do not expand the strike zone when pitchers overpower the hitters. Both of these factors will increase your pitch counts because you will be throwing 4-6 pitches per batter unless you are pounding the strike zone. If you are pounding the zone, you can stick with fastballs all day and maybe drop in a curve at some point vs the best hitter on each team.
I can’t really get into much detail unless I see what you are doing.