The Verdict: Clear Path
Your mother was Thai when you were born, so you are Thai already. Register your birth with the Thai embassy or consulate for the country you were born in.
Section 7(1) of the Nationality Act makes anyone born to a Thai father or mother Thai, wherever they were born. You are not applying to become Thai; you are asking Thailand to record a nationality you already hold. There is no deadline or age limit; first registrations in people's forties and fifties are ordinary.
If you were born before 1992, note that the old law passed nationality through the father only. Section 10 of the Nationality Act (No.2) B.E. 2535 applies the new 'father or mother' wording to people born earlier. District offices sometimes need to be shown that provision, so take it with you.
If your birth was never registered with Thailand, start at the Thai embassy or consulate for the country you were born in. The Thai birth certificate leads to a house registration (tabien baan) entry, then a Thai ID card and passport. Bring your mother's Thai birth certificate, ID card or passport. If she has died or her papers are lost, other proof can be used, and DNA evidence is accepted.
You do not have to give up your other nationality; a person Thai from birth keeps both. If you are male, Thai nationality carries military service obligations (registration at 18, a lottery at 21), dual nationals included. Ask the embassy how they apply at your age.