If you're visiting this blog for uplifting, soul-raising advice on how to handle your self-doubts with writing, please do not read this post. You will be scarred for life.
For the rest of you, let me share three harsh truisms:
1) You are supposed to suck when you first start writing.
2) Your first drafts are supposed to suck, and almost all of them will.
3) Many of the things you will write in the future will still suck, no matter how good you get at writing.
This is just how it is. Expecting to write perfect copy at any stage of your creative process is delusional, and expecting not to struggle with this craft is narcissism. What's so special about you that you should deserve to have no difficulties writing?
Get used to sucking and get over yourself. You simply cannot allow self-doubt to stop you. You owe it to the world to say what you must say--and the world can just stick it if it doesn't like it.