Why do I keep transposing letters when I write and type?

Handwriting difficulties can be purely muscular. If the connective tissues in the hand are causing a problem, then you'll find that it's difficult to control your pen. Sometimes simply trying out pens of different sizes and shapes – and even point types (felt tip, fine, medium, gel, etc.) will help you to find one that allows you to write with some degree of comfort- although this may still not guarantee legibility.