I typed out this big, long reply... and then realized it wouldn't help you. What you need to do is RELAX. It sounds like you're over-thinking things!
Get into a nice, even trot or canter when you're approaching the jumps. Do you listen to music? Think of how there's a basic beat to every song. Tap your toes to some songs and get the feel of the rhythm. It's the same thing with riding--think about keeping each stride the same as the last one. Count in your head--one-two-one-two-one-two, etc. Same with going over jumps--yes, jumps from a trot approach can be awkward at times, but once you know what to expect, you'll feel a rhythm to jumping too. Like the previous poster mentioned, gymnastics are great exercises that will help you find your rhythm over the fences too.
Anyway, don't worry about anything except keeping that steady pace as you ride to the jump, and once you're at the base of the jump worry about getting over safely. You want a basic jump position that is well-balanced. To do this, keep your weight in your heels and let the horse's jump push you out of your saddle. Close your hip angle slightly, and try to use your horse's neck for balance as little as possible, though feel free to grab mane if you need it, especially as you're learning.
After you master the rhythm and balance, the rest are just details. Piece by piece, you can work on fixing the other issues and continue to improve!