Quite a few years ago I “fixed” my (pretty bad) handwriting by essentially writing the quick brown fox jumps over the lazy dog hundreds of times. Taking a similar approach to music, I started slow and with great intention in my motions, then tried to work up my speed to a more realistic writing tempo.
I think from there, in your regular writing, you might take mental notes of any letter combinations that trip you up or could be improved. Then it’s a matter of going back to slow and intentional with those things until it’s “fixed”.
I’m not totally happy with my ‘m’, ‘u’, ‘r’, and ‘n’, but it’s been a while since I did any sort of practice.
edit: having pictures of peoples’ handwriting you admire nearby can be helpful too, I took inspiration from a few different places when I was relearning