Always thick throwing DOWN on the ball.
My son had this problem. This is what I taught him. He has thrown low strikes in the upper 90's with this teaching
For one thing, there is a saying:
Low elbow, high balls. High elbow, low strikes.
Concentrate more about where the elbow is than just the throwing hand!
Throw from the elbow allowing the forearm to whip from a larger arc.
If the elbow is low you are are eccentually pushing forward and up. This is why for one reason that when the elbow is below the plane or falcrum point of the schoulder the ball will be delivered high.
Think throwing down on the ball will help. Measure your taget low!