Before you work on training issues with your horse, consider if your horse might be in pain.
All kinds of conditions can lead to pain that would cause this behavior. Here some examples:
- bad saddle fit or screw coming through the saddle (I've seen this plenty of times)
Disclocated ribs or vertebrae
Hairline fractures (from a fall on the pasture)
Call your vet and have the horse checked out, is my suggestion. If nothing serious is found, try a chiropractor and an equine massage practitioner. (Here a bit more info about equine massage www.reinholdshorsewellness.com
Rearing is a serious sign of a) a serious pain issue or b) a serious training issue. It can be very dangerous for your and your horse.
Please don't think waiting it out will fix it.