I learned to how to stop dwelling on angry thoughts by:
1) Noticing that I am once again mentally monologing my angry thoughts.
2) Switching focus to whatever I am physically doing at the moment (washing dishes, sewing, walking, filing, etc)
3) Consciously see, hear and feel whatever I'm doing at the moment.
My angry thoughts were always dispelled by this exercise. But they would return again; within a minute, an hour or a day. Whenever they returned I dispelled them again by practicing steps 1 through 3. Within 6 to nine months I was no longer plagued by angry thoughts. I cannot describe the relief this simple routine has bestowed. It changed my life.