Hi, if you find yourself struggling to keep your heel down you should think about opening up your toes instead of driving your heel down. Pushing the heel down will most likely result in your leg shooting forwards.
Do LOTS of work without stirrups, making sure you open your hips and don't grip with your knees. When you take your stirrup back, don't change anything, try to even forget that your stirrup is there. Keep your lower back and bottom nice and relaxed.
Also, 'heel down' is not just for aesthetics, it's safety! Don't forget it; it really could save you if you become unstuck - think about it; if your heels come up, your lower leg comes back and your upper body tips forward, then you are so unbalanced that only the slightest unexpected move from your horse will tip you over the front door! Really imprtant when jumping too!
Your jumping position isn't bad, and for the height you are jumping it is fine. If you want to jump higher classes I think you should have a few more jumping lessons. Definitely need to shorten those stirrups though! And do lots of flat work in your jumping position so you strengthen your back and legs and don't have to lean on your horses neck over the jump.
Your horse does look heavy in the hands and you will need to do a lot of work to get her out of that habit. Just dont let her lean on your hands, when she does push her forwards and leave the reins, and use your seat and tummy muscles to bring the energy up, instead of forwards and down. If she has nothing to lean into she cant lean

How does she go in a loose ring snaffle? The loose ring gives them almost nothing to lean into. If you are sturggling to train her on your own, it is worth getting a good trainer who can instruct you step by step what you need to do.
Congratulations on your first show! I turn into a nervous wreck at shows! Good luck with your next one!