I start with this article because it shows you how to move into the growth mindset to help students when they get stumped in the classroom. You can use the included chart to show the students how to change their thinking.
This article explains how to get to the heart of the matter when a student says "I can't." Teachers need to find out the underlying reason for that assertion, and this article will help you do just that.
"I'm stupid" is just another way for a student to say "I can't do this." Using empathy to find out what's behind that declaration is how this article suggests you work with the student to get him or her moving on the right track.
Sometimes your students are right to say "I can't." This is an interesting take on the issue, and one you will find helpful when confronted with a student who is feeling stumped. Take a look at some of these suggestions and see which you can add to your bag of tricks.
While this resource is for parents, I think most of the advice will apply to teachers as well. After all, we're all trying to help the students be successful, and taking a step back and finding the real problem is what these solutions will help you with.