It looks to me that you are gripping with your knee which is causing the pivot effect in pushing you too far forward and your leg too far back. Work on gripping more with your whole leg especially your thigh.
You aren't releasing your horse's neck, and are resting on your hands. This has caused you to perch percariously forward on your horse's withers, and yes, it looks like you have pivoted from the knee. Drop your leg--it has slipped back quite a bit.
If you rotate just the part of you in that picture to where your legs are perpendicular to the ground, picture how your upper body would come further back, and then open your elbows to allow for more of a release, that's where your ideal position should be. Wish I still had Photoshop to do it.