Ever felt like you're not really competent to do hard physics things? Like someone is soon going to find you out? It turns out to be a pretty common thing. I wonder if it's more common amongst physicists. Anyway, Peter Coles, who blogs under an anagrammatic version of his name as, umm, erect poles, I think, has a nice post about it here.
I have certainly felt this way. The fact that I don't so much these days is probably less a change in how I think of my own abilities, which probably never go up in time, but more a realisation that I am not so different to others.