We have been working on getting used to the feeling of getting things wrong for a while with my DS6. He has some anxiety diagnoses, so we have been working with a psychologist and therapist for a while. We have used a range of techniques, from breathing exercises, "brain buttons," resetting, pushups and yoga poses. . . But we also emphasize how important it is to get things wrong, how much we can learn from making mistakes, and so forth. We will sometimes cheer at mistakes or do exercises where he is actively not supposed to do things perfectly, i.e., writing time where I don't want him to focus at all on spelling but just on ideas or volume.
Getting used to those feelings is important because I think a lot of the fervor of the meltdown is sourced from the anticipation of what DS expects the result of mistakes to be and not in any real experience (other than what he has self-imposed). The more time he spends with the feelings (in metered doses), the less power they can have to knock him out of control.
We've been told to think of it like getting into a pool. It's cold at first and uncomfortable, but if we jump right back out of the water because of the discomfort, the next time we try to get back in the pool, it will still feel cold and uncomfortable. So then we would spend the whole pool trip hopping in and out of the water and being cold. Alternatively, if we just hang out in the water for a few moments, we get used to the cold feeling, and soon we don't notice and then we are off swimming and having fun.
This only applies because DS naturally gets so upset at things that normally one wouldn't expect to be a big deal. I still try to avoid things that upset him or approaches that I suspect will not go over well when practical. For him, it's something that is going to happen until he gets tools to lessen it, lessen its impact, and steer his emotional responses. I don't know if it will help at all with your DS, but I can really relate to your observation that once he freaks out, it is hard to get anything else done.
BTW, we are home schooling and I let DS pick a few of his spelling words for each set, and he still cried when he missed *just* one word on the whole list, which was one of the ones he had picked in the first place. I just told him spelling quizzes used to make me cry, too
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