The trick to throwing lower line-drive shots is really committing to keeping your shots at shoulder level throughout the throwing motion. From the pull through to the follow through make sure that the nose of the disc is slightly downward. You must focus on driving the disc forward, locking your wrist in a downward position and following through in the line of the shot. This will keep your drives at lower trajectories and right down the middle of the fairway.
If you really want to work on gaining distance then is sounds like you need leave the constraints of your home course and play other courses that allow you to open up on some drives.