Make the sound /s/. Where is your jaw? Lower the jaw intiny increments. How far out of position does your jaw have to be before the /s/ is affected? Try this with all your sibilant sounds. Jaw stability is crucial.
Did you also know that every phoneme is produced by stabilizing the tongue in the back of the mouth?
Key elements for producing /s/ with mature speech is that the tongue is in and stable, while anchored at the back lateral margins. A frontal lisp is demonstrated by tongue out and unstable, while not anchored at the back lateral margins.Also, the jaw is always high on the consonants and lowers when producing vowels.