Has your instructor taught you the 'correct' way to use your legs in rising trot?
I have had a student come from another instructor, that had absolutely no idea how to use her legs in rising trot and would end up on her toes like you.
Basically, as you sit, your lower leg should 'hug' the horse, and as you rise, your lower leg should 'relax'.
When I'm teaching a littlie how to rise trot, instead of saying 'up, down' I say 'squeeze, and, squeeze, and...'. Once you have to use of your legs in rising trot you'll find that you're in a much more stable position as you will find it extremely difficult to lift your heels while trying to 'hug' your horse with your lower leg.
Posting without stirrups can be good, but can also be detrimental as a rider can learn to use the knees and thighs to grip and 'push' themselves out of the saddle. In dressage, gripping with the knee and thigh is a no-no as you block the horse's shoulder and then end up with a tendency to tip forward with your upper body.