If I write very slowly I can write neatly, for example when addressing an envelope or filling in a form. But if I try to write at speed my handwriting becomes illegible. This is made worse by my left-handedness, which means my hand sits on wet ink as I write and deposits it across the page. Using a fast-drying biro usually stops this happening.
I don't find it particularly embarrassing or much of a problem as I can avoid handwriting for most of my life, and when I do need to I can write neatly - it just takes ages!
I'm sure it's just a matter of practice. If I tried consistently to write neatly then I would probably learn to do it more quickly. But honestly, I can't be arsed.
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moto