An error that gets you understood stops being corrected by anyone, including you. It then gets practised. Every time you produce it, the wrong version banks one more repetition than the right one, and at some point it becomes what your mouth does without consulting you.
That is fossilisation, and it is the ordinary fate of adult second languages that reach usable and then stop being examined. It is not a sign that you learned badly.
Why it will happen to you specifically
Three forces line up and all of them are pointed the same way.
You are understood, so the communication succeeds and there is no feedback. Nobody will tell you, because correcting you costs face for both of you: that is the week 4 article and it does not stop applying as you improve. And your own monitor is deaf to it, because you hear what you intended rather than what you produced, which is the same mechanism as the tone drift in week 9.
What reverses it
Not more input, and not more conversation. Both of those give the fossil more practice, which is why people plateau in exactly the environment that should be teaching them.
Two things work. Recording yourself and listening the next day, which is the only route around your own monitor. And explicit, slow, deliberate re-practice of the specific item, enough times to outweigh the wrong repetitions already banked.
That is unglamorous and there is no substitute for it. The course spends a week here because a fossil that survives month 6 will survive year 10.
What to listen for, in order
The tone on the last syllable of your sentences. That is where French phrase stress lands and where the particles live.
The three-way stop contrast. French has two labial stops where Thai has three, and if your ear is still collapsing ป, ผ and พ into p and ph, your mouth is too.
Vowel length, which you are probably treating as a detail and which is contrastive and can carry the whole difference between two words. And missing ครับ or ค่ะ, which by now you believe you always say.
- ปาpaː
to throw
mid class, unaspirated
- ผาpʰǎː
cliff
high class, aspirated
- พาpʰaː
to lead someone
low class, aspirated. Different tone, same aspiration as ผา