Check your grip and release out. If you are throwing slurves than your release and grip are too much like a curve ball. Try adding a tiny bit more space between the fingers and moving your release/spin of the wrist angle down just a tiny bit.
If you are essentially are going for a fastball slider with late break modify your fastball a tiny bit until you get what you want. Throw a 2sm fastball grip but use your wrist and fingers to get that spin then slowly move your index closer to your ring if you need more break, keep this up until you are happy. By accident you may come up with an entirely different pitch in between what you are looking for.
If you want it to be closer to a fastball than a curve than start with the fastball. I know this isn't technically a slider by grip but I throw mine with a 2sm grip and they go hard, however the negative is they aren't going to cause batters to buckle their knees chasing out of the zone.
It's all just what you want the pitch to do.