The pilot reported that when the tail wheel touched down during landing, he felt a "severe oscillation" which prevented him from maintaining directional control of the airplane. The airplane veered off the right side of the runway where it contacted a ditch, collapsing the left main landing gear. The pilot reported that he had always been able to control the airplane in the past with the tail wheel shimmy by using the brakes. Inspection of the airplane revealed the tail wheel clutch locking assembly cam was worn. In addition, the anti-shimmy friction spring was tightened down beyond limits.