Other posters are providing advice about writing from the shoulder, and writing slower etc which I would agree with. The other thing I would mention is: how tightly do you grasp/hold the pen? Try relaxing the grasp a bit.

(I've noticed this with students I support. In the UK the public exams known as GCSEs and GCEs are written papers, and some adult students who need to take them have similar fatigue problems).

Edit: another poster has mentioned grip