Here are my hypothesis for your hand writing issue:
Maybe in your childhood your parents changed your prefered hand, so maybe you were left handed so these kind of changes can cause some problems of hand brain coordination.
More Exiting predictions are now beginning: You may have Asperger (more probably) or autism syndrome which can explain some other things about you also :) It is known that these kind of individuals can have problems in hand writing. People can have light level of asperger which can also make these people more original.
My last point there can be a problem in the eye like Metamorphopsia which one of the reasons is a kind of fungus as a result of tobacco usage. ( I am not into this argument much)
My real last assumption is maybe you really lack of practicing hand writing at school because of the style of the education.
Lastly my question for future deductions, did you learn speaking at the normal ages or any kind of relay happened? Normal periods are at age 1 start to speak a word, at age 2 two word usage begin and at 3 years of age sentence structures appear.
A very thorough analysis but I am pretty sure the only possible answer is the lack of practise. I haven’t written in cursive since it was no longer required in the 3rd grade if I remember correctly.
I learned to speak and walk and all that quite early, and my parents did not change my hands. And what comes to Aspergers or Autism, I’m sure I’m somewhere on the spectrum. But I don’t think it’s good to try and find issues where there really isn’t any, people have different traits and that is that.