+1 with Platypus and aeh.
We had this challenge with writing for my DS, with him resisting strongly and performing at a very basic level. What worked was to slowly build the skill at the pace he was able to manage - in this case, through the appeal of cursive. Early sessions were painful for everyone involved, but daily practice of at least 10 minutes was required.
I positioned the practice as learning a skill that many people need to be taught. We sat down together every day, and he was rewarded for effort if he completed 10 minutes of work (e.g. a TV show, playing hockey together, etc.) The goal eventually grew to 15 minutes per day, but no more. I would sit side-by-side with him and do the work at his pace alongside him, demonstrating the skill and coaching through frustration as he hit roadblocks. When he engaged in negative self-talk, we took a pause and re-framed the beliefs. Now, if he makes an error, instead of saying, "I'm so bad at writing. I'll never be good at this", he'll say something like, "That didn't turn out how I wanted it to. Let me try again." (Actually, his writing is now gorgeous and fluent- an unexpected strength - which makes the experience feel less painful on the other side.)
These are life skills - not everyone comes pre-loaded with the same software, and we sometimes have to hack the solution. I know we're talking about different specific challenges, but the behavioural and emotional consequences to the children feel similar. Hope this helps, and good luck!
