My lymph node has been swollen for a year and the swelling goes up and down. its located on the mastoid process?

See doc: Have an exam. Some nodes can be chronically reactive over years. Blood counts, exams and if needed a biopsy can even be performed with chronicity. Best wishes.

Dermatitis: If you truly have a lymph node on the mastoid process, it is most commonly draining from the skin of the scalp. You may have seborrheic dermatitis of the scalp. That is a possibility only. A lymph node usually gets inflamed from local inflammation and the closest area is the skin of the scalp.