For almost three years (90% of time) i have had a large, painful knot in my upper neck (lower head). the pain is extreme at times. nerves? muscular?

Muscle spasm: 20's male with knot-like sensation in upper neck likely has chronic muscle spasm. If symptoms are unilateral then chronic head tilting is to be considered. Other associated findings might be asymmetric shoulders with underlying scoliosis and even pelvic girdle asymmetry due to a unilateral SIJ hypermobility disorder. Osteopathy, massage, cannabis topicals, and acupuncture are considerations.