My handwriting was awful too until I left school. Each teacher in our primary school encouraged a different style and each one found fault with mine.
When I started to work in a Bank aged 15, I was given a copy book to work on in spare moments, mainly for training in the long thin numbers which were essential for hand written ledgers. Within a month or so I was turning out beautiful, legible script. Later, I adapted this approach with some of my trainees whose handwriting was appalling - simply choosing a desirable font and printing out sample sheets from the computer with space for their copies. Doing the same now for the DGCs. Think it is best to work with a pencil until you 'get to grips' with it and then transfer that skill to a pen - much harder to write clearly with a biro!