I dont know how long you have been riding?
But there are different schools of thought on this.
Sitting on a horse so you look like a picture in a text book is not the main goal in riding nor the most important thing to learn.
And for many of us it is simply not realisable - very hard to find a perfect rider, perfect body, perfect horse.
So whereas some RIs stand in the school and shout at you to put your heels down, (which for some people is impossible) others get on with the real business of teaching you to ride.
How you sit on the horse is far more important and enlightening than position. You should sit on your seat bones and be able to feel them move up and down as the horse walks. When the horse lifts a hind leg off the ground, your seat bone that side dips and when he puts it back on the ground and pushes off on it your seat bone that side rises.
You need to sit upright and allow your seat to move with the horse. Dont lean one way or the other. Not to either side, nor backwards nor forwards.
Let your legs hang down, dont fight gravity - and dont grip the horse. Spread your toes out inside your boots.
Riding a horse isnt about fitting yourself into an ideal pattern, straining and stiffening in your effort to match the picture. It is about being balanced and sensitive to the horse moving under you.
If position is worrying you, perhaps you should find another teacher?