The trial of a Brockton woman charged with fatally stabbing a retired school teacher in her Bridgewater condominium has started. Eunice Field faces a murder charge in the August 2010 slaying of 62-year-old Lorraine Wachsman. Witnesses took the stand Wednesday to describe the scene. Authorities have said Wachsman was a substance-abuse recovery mentor for Field’s former girlfriend and Field blamed Wachsman for driving the couple apart. Prosecutors said in opening statements that Field committed a ‘‘cold, calculated, and planned’’ murder. But defense attorneys are arguing insanity.