Past keeps on haunting if it is bad and emotionally laden, More so with females. Forgetting is an automatic process. You can speed this automatic process by making her present pleasurable. If she is ruminating her past, do not nag her for not coming out from her past. On the contrarily you can make her aware that at some point of time she has to decide that how long she wants to suffer. Understanding like this can take her attention to present and she can hasten forgetting process. Psychotherapy and self hypnosis may help.