You won't get correct flexion until you can ride with an independent seat and not pull on your reins.
Try riding with your little fingers hooked under the velcro at the front of your saddle pad, or on a monkey strap if you have one (piece of plaited binder twine works well). This will give you enough movement to follow the horse's topline, but will restrict you enough to know when you're pulling or throwing the rein away.
Your seat should be strong and balanced enough to never get tipped forward, so you have complete control over where your arms/hands/legs are at any one time. Try some lunge lessons without reins and stirrups to assist in developing the required degree of balance and core strength to achieve this.
Between these two exercises, you will train your body to do most of the work, and your hands will stay quiet, not having the need to move around. At the moment, it sounds like you are relying on your hands to do the work of your body.
Flexion cannot be achieved by pulling back on the rein, you will create too much bend in the neck, the jaw and poll will lock and the purpose will be entirely defeated - as you've already realised ;)
As DXD said above,asking for flexion is more in the wrist and fingers, than moving your whole arm. However, flexion won't come until the body is supple, and the hind legs are active. Concentrate on getting very good hind legs, develop a substantial and effective half halt, improve your seat and outside rein contact, and you will find that the flexion will come automatically. You may occasionally need to give a little reminder, just a tweak of the fingers to feel the mouth a little more, with the inside leg on, but nothing more than that.