Well you could try my method. Buy the Pilates book, put it on your nightstand or under your pillow, feel great the next morning!
No really, it sounds like you would benefit from some “professional” exercise starters. With your back history you don’t want to be the do-it-yourselfer. Do you still have the stretching exercises I sent you way back when?
For under saddle work, I suggest you do your walking, trotting warm-up totally in two-point. Don’t let your butt touch the saddle for as long as you can stand. Grab a little mane if you have to. Be sure not to pinch with your knees and do let the weight fall down the back of your leg. Do your transitions from this position and everything. Not only will it secure your base of support in your lower leg, it will strengthen and secure your upper body.
Your finishing picture really doesn’t look bad. I wonder if you are throwing yourself to far forward as you jump. Unless you are doing the Puissance Wall 
, from your two point the horse should be rising to meet you and you should not have to close your hip angle much more.
Good luck! 
“The older I get, the better I used to be, but who the heck cares!”
