Fixing my posture.

I have lordosis of my spine. I have a severe arch in my back. Never has it caused me any problems. But now that I am riding a horse, I notice that my posture is in a slouch when my tail bone is placed correctly in the saddle. Its nearly impossible to sit with shoulders back. How do I sit correctly when my body is not made that way???? I heard go to a chiropractor and get months of posture treatment, but I do not have the money to do this. Do I deal with riding in a slump? I feel so tacky when riding with my friends who have such great postures. HELP. BTW, I have a KMH and I ride western saddle.