So I have the age old problem of being a self taught jumper, and my leg flies backwards. What are some exercises, both on and off the horse, that help with this?

No stirrups is definitely a big help
No stirrups and off the horse you can do heel stretches.
Jumping without stirrups... helps to keep you lower leg still and heels down.
Squeeze with your thighs and Lower legs
When you go into your 2 point seat push down a lot with your heels so that your leg stays forward
My trainer always tells me to "put my feet on the dashboard" now obviously I don't put my feet up high or anything but sticking your leg out in front to the girth will help keep heels down, leg forward and feet in the stirrups. I know when I et my leg fall back my right foot tends to slip out so I have to really concentrate on that
drop your irons ;) and it's caused by pinching with your knees & not using your lower leg properly
I usually do a lot of bareback work, over jumps that you would jump if you were in a saddle it helps alot and you position gets better as you don't rely on the stirrups as much
I had a LOT of trouble with this, and I tried everything - with a lot of core strength I was able to get it okay sometimes, then I got a new saddle and raised my stirrups two holes and voila, easy peasy.
So it may be your tack, but might be something else.

I also found concentrating on having my toes up rather than heels down helped. And to concentrate on having a firm calf hold and never letting go of it.
My instructor tied my stirrups to the girth with a stiff, yet flexible band to keep them in place.
I used to have the same problem until I found this trick.... think about when you go to 2 point "put your heels on the girth " it will help you keep them forward
I always think of gripping with my lower leg and not knees, this gives you a wonder surface area to cover on the horse and should stop your leg sliding back. I always think of the knees as a pivoting point, when they tighten, your lower leg swings back. Also keeping your heels down will help
Focus on your shoulders being behind your pelvis and when you go over the jump just bend your hips and release
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