I’ll add my unorthodox slider method as something to try if you get desperate. I literally gripped a four seam fastball, middle finger on the imagined prime meridian of the ball. I lifted my index finger then pictured middle finger cutting the ball in half like the iPhone game Fruit Ninja. Obviously this requires a little wrist pronation. Just do what feels natural to impart the right spin on the ball. It was so second nature to me that I’m afraid I will have a hard time explaining much more. This is the pitch that made my dad stop catching me without protective gear.
Started this around 9th or 10th grade and it got me through high school, Legion ball and college as a standout pitcher on each team.