This is a big one. One of my friends would constantly berate himself like “I’m such an idiot”, “I’m a dumb old dinosaur” anytime he couldn’t fully grasp a new concept. Over time I could tell that it was actually making him less likely to give himself a chance at understanding. And he truly is a smart dude. One day I called him out on it and he just stayed quiet, took a deep breath and said “you’re right. I’m not an idiot, just need time to understand”. He no longer says it and now embraces a mental challenge.