You have been given great advice so far.
The neat thing is that although you have been given lots of different advice, each of these work together to cause the same thing. So working on one will influence the other.
The only thing I haven’t seen addressed is foot placement on your stirrups and how you angle your foot across the stirrup bar.
If you move the stirrup closer to the center of your foot (instead of just barely the ball of your foot), your heel won’t be able to drop as low, which will give you the shock absorption through your leg. You still want to keep the weight in your heel, its just not as dropped.
Changing how you angle the stirrup across your foot will change the part of your leg that is making contact with your horse.
You can also adjust how you angle your foot to the horses body. Riding with your foot parallel to the horses body or riding with it slightly turned out. This changes the contact points as well.
Experiment will leg position and stirrup length in the ring. If your horse is reliable in the ring try riding the two point and half seat without reins. Start with a light rein contact until you build up some strength. Do the up,up down, down exercise. Up, up, down, up, and up, down, down are also great for forcing you to find the balanced point in your two point and half seat. Get your mom to call out the exercise and change it up every 10-20 strides. Do this in the trot and canter so that you can figure out how to SIT when you need to.
Once you can do this, when in the two point, play around with influencing your horses stride and direction with just your legs and seat while in the two point and half seat.
A grab strap properly adjusted will help you to have something to balance on when you need it without grabbing and hanging onto your horse’s mouth. Some people like a martingale yoke to grab, but I like a stirrup leather.
