I would recommend buying Joan Wulffs Dynamics of Fly Casting Video. It is set up in sections so you can become proficient in each part before moving on.
You will have to practice when you ar not on the water fishing as fishing and learning to improve your casting do not mix. While I can not find your casting problem without seeing you It sounds like you have a tailing loop which can be caused by trying too hard and applying power in your cast too soon. Your stroke should start out slow and accelerate to a stop. Make sure you have all of the slack out of your line before you cast. Following is my my attempt to help someone else with their casting.
Your rod tip should be near the ground before back cast,and your wrist ought to be bent down so the rod butt is parallel to your casting arm. Lift the rod tip in a smooth acceleration as you rotate your wrist to a straight position into a squeeze stop. When your wrist is in a straight position the rod butt should make a 45 degree angle with your casting arm. Relax your grip allow your casting elbow to move up a few inches which allows the rod to drift back. You will drift back farther on longer casts and hardly at all on short casts.During the drift time your line unrolls behind you. The forward cast should begin just slightly before or just as your line straightens out behind you. On the forward cast your elbow moves to its original position as you move your rod hand forward in a smooth acceleration rotating your wrist into a squeeze stop. At this point your wrist should be bent down (rod butt parallel to your casting arm). Relax your grip and allow the rod tip drift down to near ground level. Try to make your back cast stop at 1 o'clock and your forward cast stop at 10 O'clock.