When my daughter, who takes after my hubby & I with her perfectionism, gets frustrated her craft projects turn out poorly (according to her high standards), I ask her "Have you ever done it before?" The answer is always "No." "So," I tell her, "then it's the best you've ever done." Slowly she's getting the idea to celebrate her attempts rather them condemn them.
Whether or not it's your first write or your third rewrite, if you look at the facts, it IS the best you've ever done simply because you've never done it before.
Sometimes you may need to get a bit creative: "This is my best picture book today/this afternoon" or "This is the best story I've written on South American dragons." When you narrow your focus, you can honestly pat yourself on the back for doing the best you could. Will it be the best you can ever do? Who knows?!? That forward focusing viewpoint can only lead to frustration.
Today is the day you are living in and your current project is what you are focusing on and since this is all you have truly in your control, it is the best you have ever done!
So when those doubts come creeping in and you can't help but think of someone else, stop yourself and acknowledge the success of what you are doing, right here, right now.
And feel free to apply it to all areas of your life. Heaven knows you need more acceptance and approval and less criticism! Enjoy it!