Get a notebook and write down everything that happens surrounding the tantrums. What preceded the meltdown, what triggered it, how did your DC get out of it? After you've written things down, you'll start to see patterns. We were in a similar situation, and after a while realized all the tantrums were about food. We found out DC had a severe allergy to milk, and we'd been feeding DC milk every single day. DC craved milk, screamed for it, and when we gave it to DC, screamed even more. We stopped feeding DC milk for a week, and the tantrums disappeared. Not saying this is your child's issue, but it could be something similar. DC also had problems in circle time at school, and we found out DC was allergic to dust. DC sat in a chair for circle time, and that problem went away too. Don't go too deeply into psychological reasons, OP. It might be something physical, and in my experience, it usually is.

This happened to my child after we moved and he had trouble adjusting to the academic and behavioral demands at school, which were unreasonable. Try to reduce stressors at home and find out how they are treating her at school.

My dd has been like this since birth so I feel your pain.

Is her behavior much better at school than at home? (If so, that is a really positive sign! It shows she knows appropriate behavior, can hold it together at school but is out of self control by the time she is "safe" at home).

18:40 I feel for you and can completely relate. My daughter to gets really upset and has a hard time calming down. She goes through periods where all seems fine and then suddenly we are back to the meltdowns again. She is 8 and is in 2nd grade. We can't pinpoint what sets it off either and just like your son, she knows what she can do to calm down (breathe, walk away, think of something positive, change your thinking, etc.) but in the heat of the moment all those things go out the door and she can't seem to calm down. She has ADHD. She has been doing CBT also with a therapist but again in the heat of the moment she melts down. She is also doing a social skills group w Dr. Kelly Obrien too. This seems to be helping a bit and they work on changing their thinking, etc. hopefully with time she will come around and realize it doesn't solve anything and drives ppl away. In the meantime, patience is key - it is so hard I know. Just know you are not alone!