OP here. Thank you! I'd appreciate any more suggestions.

Try medication.

I do "and then what" with my kid when he gets this way. Reassurances don't help stem the panic. He says "it was an assessment and I totally messed it up!" Instead of saying "no you didn't, it looks fine," which is true, I say "Hmm. Okay, well if it is totally messed up, what happens?" "My teacher will think I"m stupid!" "Huh. Okay, so she is going to think you are stupid. What will happen next?" And just step by step you walk through it, until the kid sees that each of those bad things happening are things he can survive and manage. Doesn't work during a meltdown, but it does work for my kid at that point when he's just having catastrophic thinking.

My kid gets this way sometimes too. It isn't easy. Learning to take a few deep breathes, the stop himself and think, "It's all good."

Also, when it comes to assessments and tests, they are tools to see what he knows so teachers know what to teach him. They aren't to measure his personal worth or to punish him with bad grades if he does poorly. Have this conversation with him when he is calm. Repeat it a few times until he gets it, and coach him through whenever he feels even slight pressure on an assignment.

DS had a really hard time this morning because he has a writing assignment due and worries that it is terrible. Since it was couched as an "assessment," he feels immense pressure that it should be perfect. He wants to start again, but he would run out of time and that would mean his assignment would be incomplete.

Engaging him in his circular thinking aggravates the situation and CBT techniques fly out the window when he is in distress. What helps your child? Any suggestions for positive self-talk?