No one sets out to suck. You try your best and you see where you end up.
But what if you flipped it and tried to do the worst story possible. Toss out all your knowledge and common sense. Ignore what you like or what you believe in.
The only rule is to stick it out until you finish it.
Once it’s finished, look at it. You have a completed a full story, albeit a bad, broken, and ugly story. And, somewhere inside of you, you know you could have done it better.
The trouble is that you never let it out because of fear.
So, quit worrying about perceived failures and write because the Greatest Fail would be not writing at all.