This is not the place to ask this. Many of the posters do not have medical training, or specifically sport-injury/rehab training, and those that do, have not seen your back to be able to provide specific advice on what stretches can help.
Back injuries are incredibly serious, and being thrown from a horse could be anything from a simple sprain, to a shift in spinal alignment, to a direct fracture or damage to a disk. Obviously, this is not me saying any of those happened, or none, I'm sure your doctor already knows the extent of your injury.
These are questions for you to ask your doctor or, if he thinks you are appropriate to stretch, and wants to rehab you for sports/functional purposes, a therapist that he refers (or one you like, who is in contact with the doctor's office to know exactly what your injuries are).
Anyone who offers you advice for this from a forum, whether here, reddit, facebook or tiktok, does not know your situation and by that alone is not qualified to provide you advice. Whatever advice they give may help, but could also cause bigger problems. And back problems are not something you should take that chance with.
Even someone saying "let it heal before you do anything" is not qualified to say that. They may very well be correct, and you need rest and ice, or you may need to slowly stretch it out, while heating in between. Or something else entirely. The point is, we do not know, without examining and working with you directly.