First of all, be PROUD of your Native American heritage!I am, and have mostly still black hair (at age 72) and high cheek bones to prove it.
My great-grandmother was born on the Alabama-Coushatta Indian Reservation in Texas.It was not burned out, as such, but what they called the 'long house' burned and that's where all the records were kept.
But not to worry!Indian agents were paid BY THE HEAD to keep records on all the Indians under their supervision and they kept GOOD records.These records are in Washington, DC, commonly referred to as the "Dawes Rolls."You may have to pay a fee to get the info you need, but do your homework diligently so you don't have to pay more than you should to get the info you actually need.
One more thought:Go to www.google.com and search for "Mary Williams, Choctaw/Cherokee Indian" and see what comes up.You may have to look at 30-40 websites and NOT FIND ANYTHING, or you might luck out and find exactly what you are looking for.
Genealogy research is (1) never-ending, (2) time-consuming, and (3) yes, a mystery.Keep a notebook in front of you.Every website you look at that has ANY KIND of info should be noted, i.e., (for instance) http://www.cherokeeindians.northcarolina.comhttp://www.cherokeeindians.northcarolina.com .(This is just a made-up website but you get what I mean!)When I first started research, I would find something and NEVER be able to go back to it.So keep GOOD records!