I know, I know. I promised to give you my advice on how to deal with bad wizards, and then I didn't do it.
But here it is: When your brain starts in with that hideous self-talk, telling you how stupid and talentless and useless you are, visualize a stop sign. See it. Say "STOP!" to yourself. (I recommend doing this silently, or people twitch.) Then, force your brain to go to a happy place.
Back in my radio days, I periodically made mistakes on the air. Hard to believe, but true. I would use this technique to stop beating myself up. It took a while, but it finally worked. Now, when the negative self-talk starts, I stop it. Fast.
Never let a bad wizard win.