Well I am really tired now, but I think he said that the hard sign means there is a sort of break and so it sounds different... ш - щ is difficult simply because this is an "artificial difference" for me, just like adding "the" and "a" is for you. To me, this is more or less the same sound and it seems a bit picky to differentiate between them. For words that I have memorised, I have obviously memorised the correct spelling and I don't mix it up once I learnt it for a particular word. But it just feels unnatural and complicated.