As a student of Xuan Tong, Teacher Lin Xi talks about writing posture. The key point is of course to relax the whole body and not to struggle. Being particularly tired is due to the incongruity of tension.
Stand with your feet about shoulder width apart. You don’t need to bend your waist. The table needs to be higher. Ancient desk paintings were relatively high. The shoulders, elbows, and wrists all need to be very flexible, with no spots where strength can get stuck.
Teacher Lin Xi holds the pen a little higher. You can lower it a little first, starting from one-half of the pen holder, and slowly move it up, until you feel comfortable and able to control the pen.