Sorry to hear it but if you cannot obtain proof that she is a Thai national then you probably won't be able to. Unless you do the full process as if you never had a right of nationality to begin with.

Send your birth certificate and mother proof of Thai nationality to your nation's Thai Embassy for a Thai birth certificate registration (If out of country) and then just go to the local Amphor office when in Thailand. Much faster than Doing the whole process in Thailand as they will have to mail paperwork to you country of nationality told me about 3 month for my kids.