If the *design* of your position is correct in the approach, and you end up in the pictured position after the horse takes off, it means that you have locked one or more of your joints, so instead of absorbing the thrust of the horse's jump, the thrust acts to push your locked up self into the position you see in photo.
To fix this it's important to understand the difference between design of position and body mechanics. Your two point can appear correct in the approach, but if your ankle, knee and hip do not flex and your angles close as the horse pushes off and rounds, well, this is what happens.
I agree with the previous posters that pinching with the knee is a primary culprit, but I think you need to look at all the joints.
First you need to make sure your ankle, knee and hip are working correctly as shock aborbers and identify times when they tend to freeze up. Next comes lotsa lotsa work without stirrups and maybe some work bareback, followed by lotsa lotsa small grids and gymnastics ridden without reins for you to really concentrate on following the horse's motion correctly with your hip.
I have some more exercises to suggest, but I'll have to post them later.