If your fork ball is really droping (not just falling from lack of speed) I would certainly keep it. Batters saw Bruce Sutters ( yes Sutters was a splitter-its all forkball variations to me) , Elroy Faces, Lindy McDaniels for years and chased them into the dirt never connecting. If your forkball is naturally diving when it gets to the plate BY ALL MEANS KEEP IT and perfect it. You have an out pitch in the making.
That being said, I would also keep working on your curve. This will be a big advantage when you face other left handers. Once you master it, you might also "back door" a few righties with it too.
At this point you have enough pitches. Look at Steves article. 3 Pitches guys, fastball, 2 plane breaking pitch, change-up. The Reds teach 4 pitches and add the slider-a real slider not a cutter, but that is when a guy reaches the Bigs. Unless you are a true junkballer, Thats all you need!
Guys that have 8 pitches generally are making up for a mediocore fastball, or learned to pitch that way as a style choice. Ian