I'd forgotten about my speech impediment, until I was obliquely reminded of it just now by Z.
Until I was six, I couldn't pronounce the 'th' sound. This was a serious problem, apparently, because I was sent to elocution lessons at Cranleigh Road school, where I rapidly learnt the trick. You just put the tip of your tongue behind your top teeth. (A phonetician writes: there are different vocables or phonemes, nasal or otherwise, of this combined consonant, as in 'there' or 'anathema'.) (An elocution pupil writes: thuck off!)
So the cure worked, but the damage had been done. Being told, before I was six, that I couldn't speak properly must have put me off the idea of speaking. Once I'd got the hang of it, it probably took me quite a while to become brave enough to try it.
(I also couldn't rrrroll my rrrrs, and still can't, but that's another tongue twister entirely.)