Good job trying out something new. Here are some tips though:
1.) You are waaaay too far out of the saddle and ahead of the motion, thank goodness your horse doesn't appear to be a head thrower as you would be missing your pearly whites. Your centre of gravity shouldn't be further ahead than the cantle and your butt doesn't need to be so far out of the seat. Think of just folding at the hips and bringing your butt slightly out of the saddle.
2.) Your leg has remained perpendicular to the ground - well done on that! I would like to see your stirrups raised a hole or two, your heels down a little and your toes pointing forwards.
3.) It seems like you are gripping at the knee, don't worry that is what most people are inclinded to do when they start over fences. However that is the last thing you want to do as it interferes with your lower leg position. Rather, think of anchoring your feet at the girth, opening your hip angle and barely touching the saddle with your knees, this will make it much easier to keep those heels down and toes forwards.
4.) Open your chest a little by not rolling your shoulders in too far, this helps create a more stable position and supports your back and neck.
Great work on looking ahead and well done for grabbing the mane so that you don't jab your trusty steed in the mouth. Good luck with the jumping, looks like you guys are having fun!
All horses deserve, at least once in their lives, to be loved by a little girl.