Thought i'd throw a few things in for you.
Before you ride, but still sitting in the saddle, take feet out of stirrups and stretch you legs as far as you can this may help you to ride longer, I see too many people not riding long enough, this will change you posture as well, becoming much straighter.
It may be the pics but your saddle in all pics seems to far forward this will hinder your horses gait.
Did notice in second pic your horse is not ingaged, this means his hind quarters are not working with his front, causing him to drag his back feet on occasions. Would suggest if you have not already get some leasons, so you and your horse, will be more as one.
If your flat work is not ok, then neither will you jumping.
I know this is a lot, but it takes hard work and dedication to get there.
And remeber to have fun weather competing or not.