Myth - You should contact the ball waist high on all of your shots.
Truth - What about high balls? Now, if you can get your opponent to agree to stop running you all over the court, hit the ball with the same trajectory every time, and no high balls, contacting the ball waist high could be a winner!
Unfortunately, this does not happen in a real match. You must learn to hit balls ankle high, knee high, waist high, shoulder high, and even head high! Yes, hitting somewhere between the knees and the waist may be the most comfortable for most players. If you can do this, fine! But do not fit yourself into a rigid mold that says you must hit the ball waist high or else you are not playing good tennis.
Your goal is to play the ball at whatever height in any given situation. Stop chastising yourself every time you do not hit a ball at your waist and you will come closer to thinking and playing like a pro than ever before. Just play!!! Time and practice (repetition) will eventually teach you to hit balls at all different levels and hit them well. I guarantee it!