The Verdict: Clear Path
You have most likely not lost Thai nationality at all. Ask a Thai embassy or consulate what your record shows, then renew your Thai ID card and passport.
Section 22 of the Nationality Act says a Thai who naturalises elsewhere loses Thai nationality, but since 1965 not one involuntary loss under it has been published in the Royal Gazette. In practice the Minister acts only on a voluntary renunciation, and many thousands of Thais hold a second citizenship and a Thai passport.
Nationality decisions take effect only on publication in the Royal Gazette. If nothing was published about you, your status did not change.
Two things would change this, so rule them out: a renunciation you signed that was then published, or (if your father was not Thai) taking an alien identification document in Thailand, which section 21 treats as loss.
Do not pay anyone to recover a nationality you probably still hold. If you are male, Thai nationality carries military service: reserve registration at 18 and a lottery at 21, dual nationals included.