" I never learned the language in any proper way " this is where you are so important, now here is the thing,
Most of us those who spent their childhood in Sri Lankan education system, then they already learned the English language in so called proper way more than a decade, if did higher education then another decade or half. But STILL more of them couldn't speak or write properly. see, the problem is still there.
this is where the native speakers like you are so important, because the way you people memorize the tenses, words, idioms & using patterns are quite different with non-native speakers.
" I don't think I'll be valuable as an instructor to anybody " no.. no.. never ever think like that, Remember the man who won the toast master contest last year,can't remember his name though said "I see something in you, but i don't know what it is, if you can do this for a week & two maybe.. maybe...we can find good teacher" (slightly change the quote)