[Pikebreath]

Without seeing your casting stokes, my best guess is you are likely not coming to a "hard stop" at the end of your back cast and forward casts. Casters who struggle often "push" the cast applying power at the beginning and drifting / slowing down to a stop. One needs to accelerate / gain speed through the cast ending with a hard stop applying the force at the end of the cast. Think of a two headed hammer with a nail at eye level in front of you and another nail above and behind your ear. When you swing a hammer you want the force applied at the end as the hammer hits the nail. Fly casting is the same principle.

I have heard Barry White describe it this way. You have two edged sabre in your hand... Now "stab the sky" and then come forward and "chop the chicken"' (take the head off)

Line control with your line hand is every bit as important as your casting arm / hand movements. You need to keep the line tight while casting. If you let "loose" the line while the rod is still moving, (letting go too soon or simply dropping the line ), the rod will start moving "through the line" which is simply wasting the rod's travel. You want the line moving "through the rod guides" which can only happen with control of the fly line.

You need to hang on to the loose line with the line hand through the casting stroke and only let line out once you have come to a stop. Control the line going out by letting the line slide through an "O" made with your finger tips and then grabbing / stopping the line after you have let out about a rod's length of line. Let the line feed put and straighten loading the rod before beginning the opposite casting stroke. This is a timing issue and learning to wait until you feel the rod load is paramount. I would suggest that at first to let line out only on the forward cast until you have mastered rod feel and timing.