I wear the brace because it reduces pain from my scoliosis... it's not really a cosmetic thing. There are a lot of exercises you can to do strengthen the muscles in your back, which will help you hold your position more than a brace will. I suggest doing those - I do a couple of different exercises to keep my back and core really strong, which helps.
As you get stronger, you can add weights to your ankles and hold barbells in your hands... The first one, which works your back muscles, you need to lie on your stomach with your arms out in front of you. Lift up your arms and legs and hold for 5 seconds. Relax. Repeat. I usually do 30 reps. The second one works your core. Lie on your back with your knees to your chest. Straighten 1 leg and the opposite arm (put the arm behind your head), hold, switch, repeat. Hope that helps!
May I ask what breed your horse is? and What saddle you are riding? I have a TB that has wide shoulder span like a Warmblood, and I'm looking for a good jumping or dressage saddle. Thank you. You are an amazing rider and your horse has great scope!