Some years ago, I went on a "positivity" course. My sister had died, my father had died, and I'd had cancer, and a broken heart, and I wasn't, quite frankly, feeling that cheerful. Perhaps, I thought, I could brainwash myself into feeling a bit better. And so in a central London hotel, with cream walls and a blue carpet, and tables with those pump-top coffee flasks of sour filter coffee, and…