I struggle with crippling self-doubt and perfectionism, and I still write. You've gotten some really good advice from everyone who have answered already and I suggest that you read all of it carefully. I'm not going to repeat it. I'll just add that what works for me is to:
JUST DO IT ALREADY!
Do your best, take pride in your work, but worry less. And when that self-doubt rears its ugly head (which it will), you push it aside, tell yourself you'll deal with it later, AFTER you finished your writing session.
Do you really want to wake up one day one, two, ten, twenty years from now... or even worse: On your deathbed, and realize you wasted years not writing because you were scared?
Will your writing suck at the beginning? Probably, unless you're some kind of writing wonder child, but I can guarantee you that you're not going to get better if you don't write.
Go write now. Set a timer for 10 minutes, don't think, just write. It's 10 minutes of your life. No one's going to see it, it's not a lot of time wasted. It doesn't have to be good. Just write.