If you've taken a class with me, this will be familiar. Or if you've read my book, you will know why during at least once a class, I berate my students to stop offering disclaimers for their work. And I've probably even written about this here.
A little backstory: We always do freewriting at the beginning of class and, inevitably, one or more students preface reading their freewriting with something to the effect of, This isn't very good, but here goes.
Now, while we all think at one point or another that what we're writing stinks, I'm anti-verbalizing this negativity aloud. I even like to get at the thought; when you think what you're writing isn't very good, quiet the thought by writing some more, not moving away from your computer or tablet.
It's nothing new: Negative thoughts influence you, well, negatively. We need to be our own biggest advocate, most enthusiastic supporter. And when you put yourself down, it's like dousing your biggest supporter with a bucket of cold water.
I usually write on the board: NO DISCLAIMERS. In my private group, I penalize students a quarter for giving a disclaimer. I'm out to eradicate disclaimers, don't you know.