My handwriting is bad. It's worse than bad, it's criminal. Half the time, I can't read it, so I can't expect anyone else to do so either.
I used to say that it had deteriorated when I was at University, because I was making lecture notes in a hurry, but I recently found some old Uni notes and they were perfectly legible.
I am very impatient, so I write, like I do many things, in a hurry, even when I'm just writing a shopping list. I try to slow down, and my writing gets a bit better, but then I forget I'm trying to write legibly and speed up again. I've tried using all sorts of different pens, but nothing seems to make a difference – a spidery inky scrawl is no more or less legible than a thick pencil one, just more likely to be smeary as well.
I do know why my writing is illegible, and I know how I could improve it. I don't finish off the letters fully – o's don't meet at the top and look like u's, e's and i's are both tight diagonal loops, risers and descenders are too short. It's a common problem in bad handwriting – I know, because I used to try to teach students how to improve theirs. Unsurprisingly, they didn't take much notice.