I noticed my handwriting was terrible, and consciously improved it by writing slower and being more mindful of writing neatly. A fountain pen helped me slow do

I noticed my handwriting was terrible, and consciously improved it by writing slower and being more mindful of writing neatly. A fountain pen helped me slow down, but fundamentally it was just a matter of slowing down and consciously forming nicer characters until it became easy to do so, at which point my speed increased--but I retained the habit of paying enough attention to make nice characters. Deliberate practice (as in worksheets or exercises) is less important than just going as slow as you need to, to make the characters correctly, until your muscle memory builds up and brings back the speed.