"The worst enemy to creativity is self-doubt." -- Sylvia Plath
Self-doubt is the meanest of murderers. It likes to torture its victims slowly, pulling out hair, nails and teeth one by one. I was so plagued by self-doubt in my early 20s that I couldn't write more than a paragraph a year, and that's only a slight exaggeration. I thought that since Shakespeare et al., had already said more than I'd ever be able to say, and considerably better and more beautifully than I'd ever be able to say it, then what was the point of my writing anything at all?
The problem was that, despite this, I still wanted to do it. It wasn't until I was about to turn 30 that, inspired by a workshop (Instantaneous Transformation with Ariel & Shya Kane) I went on, I finally thought f*** it: I have something I want to write and, even if it's a poor example of the craft, I'm going to do it anyway. So I sat down in a room for two weeks and wrote Men, Money & Chocolate. It certainly wasn't Shakespeare, or anything approaching those lofty heights, but it made me happy to write it and more than a few people happy to read it.
Self-doubt is second-guessing the outside world. It's assuming that you have to be better and/or different in order to succeed. But you don't have to be Shakespeare/Picasso/Mozart/Meryl Streep in order to find your niche in the world.! And, since you really have no idea if people will respond positively or not to what you have to offer, you may as well assume that they will and get on with it!