I learned, as part of other neurological investigations, that I may suffer from "dysgraphia", which is actually neurological difficulty with writing that doesn't affect other common tasks (e.g. typing). Turns out it's in the sort of family with dyslexia and dyscalculia, but much less diagnosed since it is less relevant these days.

So maybe it's not about how hard you tried or a failure of learning, but that you're just not meant to write longhand and it's not your fault. I certainly felt better about my abysmal handwriting after learning some of the mechanisms behind it.