What worked for me is learning some better letterforms from some free images from the Write Now book (by Getty-Dubay) on italic cursive. It’s a different kind of cursive from the awkward one I was taught in school, and it’s a lot easier to write and read.
I think the biggest improvement in my handwriting was just finding letterforms in that book that are both easy to write but that are also more clearly distinguishable when you write quickly. For example, just putting a little curl at the bottom of my lowercase T’s, I’s, and L’s made them a lot more aesthetically pleasing but also more clearly distinct from other letters.
Once you find some letterforms like that, it just takes a little practice to rewrite your muscle memory.