Comment by mangamadaiyan

This is likely because you're writing by moving your wrist, as opposed to "arm writing". Learning to use your arm muscles to move the pencil will slow your writing down initially, but will give you more control and reduce strain over the long run.

See, for example,

https://youtube.com/playlist?list=PLrMvJfuADfNUx6s3PEFaraQP7...