Your result
Likely
On what you have said you are a Thai citizen, and this is the part of Thai nationality law that is most often got wrong - including, sometimes, by the office you will be dealing with. It is worth understanding before you go.
Before 1992, section 7(1) of the Nationality Act passed nationality down the father's line. A child born abroad to a Thai mother and a foreign father did not take Thai nationality at birth. That changed with the Nationality Act (No.2) B.E. 2535 in 1992, which rewrote section 7(1) to read 'a father or a mother'.
The part that matters to you is section 10 of that amending Act, which applies the new wording to people born before it came into force. It is retrospective. So your mother's nationality does pass to you, even though you were born under the old rule.
This is the provision to have in hand. Applicants born before 1992 do report having to argue the point with district offices, and the answer to that argument is section 10 of the Nationality Act (No.2) B.E. 2535, which is why it is named here rather than summarised.
What happens next: if your birth was never registered with a Thai embassy or consulate, that is the first step, at the mission for the country you were born in, and you should expect an older case to need more documentary work than a recent one. The Thai birth certificate leads to a house registration entry, the tabien baan, and from there to a Thai ID card and passport. Bring whatever establishes your mother's Thai nationality; if she has died or her documents are lost, other evidence can be used, and DNA evidence is accepted to establish the parental link. There is no age limit and no deadline - first registrations in a person's forties and fifties are ordinary.
Thailand does not require you to give up your other nationality. If you are male, Thai nationality carries a military service obligation, though someone who acquires Thai nationality after the age of 46 is exempt from it.