To be honest.... I think some people will always have bad handwriting
I don't have any practical solutions. I had special instructions studying teaching at Uni that I really needed to improve to pass. I guess I must have tried really hard to do so lol.
My daughter is the same. I find handwriting SO frustrating because it is impossible to get my thoughts down quick enough. Learning to touch type on a typewriter, and hence being a very fast touch typist now is my saving grace.
I can barely read my own addressed letters.
One thing that probably helped in the past was a book about calligraphy. I used to practice as a child as the pen was so nice to use, and the style of writing so elegant. If you saw the work I did with a calligraphy pen, compared to with a normal pen you wouldn't know it is the same person writing. Perhaps something like that would help?