super glue is an idea, but that would ruin the saddle.
CAPS are not for yelling here, they are for emphasizing...
so this is what i see you doing - your main gripping area of your leg is your knee. it becomes a pivot point - so that your upper body swings forward and your lower leg swings back. your angles (hips, knees, ankles) are stretching open rather than folding.
First - work on your two point. make sure that in your two point, your leg is at the girth, don't just press down on your stirrup, press a little Forward to keep it at the girth. Make sure that your hips are BACK over the seat of the saddle, not forward over the pommel as they are in you picture.
then, do this with your reins tied in a knot and your hands out to the side, pressing all your weight into the stirrups, make your knee practically loose.
then do this without stirrups a little bit - posting.
Only once you can do that WELL should you try it over a jump.
So set up a cross rail, trotting, on your way to the jump, get in two point, and focus on pressing those heels down and FORWARD. as you press your heels forward, your hips will come BACK over the saddle. that is good.
then over the jump, don't change a thing. the jump is plenty small, you don't need to move your hands, shoulders, or anything. just stay the same, but allow your hips and knees to fold more. Fold, not stretch out as you are in your pic.
then as the jumps get higher, let the lift of the horse fold your angles (knees, hips, ankles) more and push your hands forward, but your shoulders and hips stay back over the saddle, your heels stay at the girth, because your are pressing them there and gripping a bit with your calf and inner thigh.
don't be offended about going back to cross rails. its the safest way, and we ALL should practice over cross rails until we get it right!