Writing is the thing I'm most experienced at. Up until recently, it's also been the thing I find hardest. In fact, there were several years when I was unable to do it at all.
Alan Garner wrote that "There's no such thing as writer's block, only writer's impatience." I half agree with him: trying to write "seriously" when you don't yet know what you're trying to say is asking for trouble. However, it's not the only reason writers get stuck.
Frankly, unless you're supremely well-adjusted or utterly rhino-hided, simple fear can strike you dumb. If the last thing you wrote was rubbish, why should the next one be better? - and if it was halfway decent, what if the next one is disappointing? - and so on, unto paralysis.
I started this website during a period of block, because I finally realised that making (other) things was much more fun than Not Writing.
Poetically enough, after a couple of years I suddenly found the nerve to add a Poem Of The Week blog to the site, committing myself in the process to the discipline of daily early-morning writing. My output has been remarkably steady ever since.
I've realised, belatedly, that creativity is not a meagre thing to be hoarded and cosseted, but an abundance that simply asks to be trusted.