Omg, what an adorable pony! What's it's name?!?!
I actually used to have this problem with my back, arching it too much. It's almost like an old "style", but we like flat back now. A good way to think of it, is instead of thinking of less arch in the back, think of sucking in your belly button. It feels really really strange at first, but with time it starts to feel natural.
Like the others said, you are ahead of your horse, wait for him/her to jump up to you, instead of getting ahead of the motion. If you feel comfortable with this, and you have a trainer/helper around..... drop your reins though a grid and don't hold on with your hands. This will make you balance so fast, because you can't get a head of your horse, or it won't be pretty and you will be scrambling to grab onto something. But if you are confident enough and your horse is sane enough, I really recommend trying it, it will help a lot.
Good job keeping those shoulders back and looking up to where you are going.