It looks like you pinch with your knee, and therefore your lower leg swings back. You also appear to be jumping ahead of the motion. Remember to sit back and wait, and barely move –*just let his impulsion lift you out of the saddle. Also, you're dropping him over the fence – keep light contact at all times; don't just let the reins fly free. Your upper body is collapsing over the jump, don't lean over when you're jumping at all, stay upright and supple. When you lean down over jumps, you throw yourself up onto your horse's shoulder and make it difficult for him to jump. Think of jumps as just another canter stride.
I think that it would be in both your and your horse's best interest to not be jumping at such a high height. Go back to 18" cross rails, and just practice riding a jump like it's a canter stride, and nothing more.
It may help you to put your hands a little farther up your horse's neck –*not to where you're leaning over and throwing him on his forehand, but just a bit up to help you not drop him over the fences.
Doing stirrupless and two-point work will help strengthen your leg, so lots of that is good!
Great job, by the way –*I wouldn't guess that you've only been riding for 6 months! :)
Never underestimate the power of human stupidity. – Robert A. Heinlein