Sliding Doors (1998)

This movie was about a woman named Helen who comes across a parallel universe. From an incident at the subway she becomes two different lives. In one she finds her fiance cheating on her, leaves him and finds another guy, James. When in her other world she goes home and never knows that her fiance is cheating on her. Eventually the two worlds collide and her new life dies. While in the other life she leaves her fiance and finds James again. This movie was the perfect example of parallel worlds.