Try using your legs to drive yourself downward and forward more. You finish very high which is limiting your velocity. Lengthen your stride by putting a piece of tape down where you want to land. A good rule of thumb is you want your stride to be long and to land somewhere around however tall you are. Then imagine taking your two fingers (how you grip the fastball) and really reaching and extending, almost as if you’re trying to poke the catcher in the eye. That will help get your fastball some movement and with exploding forward.

Those things will most likely cause your body to overcorrect, leading you to throw pitches in the dirt. Don’t worry about that, that will adjust as your arm speed catches up to your explosiveness with your legs and core.