Shorten your reins. That'll bring your hands out in front of his withers, rather than behind your pommel. It will also give you a better feel of his mouth.
I'd also think about dropping your stirrups a hole - in a couple of the pics, they look really short, and in the others they look okay, but I think you'll have an easier time if they're a little longer. It also looks like you put most of your weight on your toe - I see that you do some bareback riding already, which is great. Start doing some of your schooling bareback, or stirrupless, to help you be less dependent on your stirrup.
Your posture is very nice and relaxed - good job! That's something I struggle with...I'm like the Hunchback of Notre Dame when I ride.
Anyway, I think once you've fixed your leg position you'll be much more effective, and you also won't be tempted to throw yourself in front of the saddle at the posting trot, as you are in the last picture. It'll be an easy fix if you put the time into correcting it!
I agree with lengthening your stirrups...your overall posture is great! It looks like your saddle may be a little too small for you but its kind of hard to tell in the pictures and where is your helmet ha ha You are definitly on th right track... keep us posted try lengthening your stirrups and take a few more pictures!!!
It looks to me like you have very long thighs. This can make it hard to find a saddle that fits you correctly. I do think dropping your stirrups a hole or two is a good idea. That will help you get away from your chair seat, but you may still struggle with that until you can ride in a saddle that fits you better.
You have good basics but need to refine the rough edges now. Bring your elbows back so your upper arms hang naturally straight down from your shoulders, and keep your elbows bent about 90 degrees so you have a straight line from your elbow to the bit. This will let you have softer contact with your horse's mouth.
I love your straight back and your happy, relaxed expression, and I especially like the helmet, which I hope you're now wearing all the time.
When you post you dont want to go that high. You actually dont have to go up much at all. Let your body relax and work with the horse. Soft up and move with his body.
Deff let the stirrups down they look too short for you.
Have a little bend in your arms to have your hands more out and infront of you.