Inability to write & poor handwriting is unfortunately a modern day phenomenon. Ages ago, hardly anyone had handwriting problems. Our teachers were not as "well-qualified" as today's teachers - our primary school teachers had no degrees.
From my observation, once you have learnt it incorrectly (from Yr 5 or 6 onwards), it is extremely difficult, if not "impossible" to correct it. So the best time to get it right is when you are starting to doodle, write & draw (I suppose that is about Yr 5 or 6). Once you have not learnt to write it correctly, and your poor style is set in concrete, then it is almost irreversible.
I'm sorry to hear that you have this problem in the extreme.