Psychological Horror Suspense Movie No Later Than 1972
No earlier than 1969 and no later than 1972 I saw a strange movie. A young woman, possibly recently released from a mental institution, was sent to an isolated house in the country to recover. She witnessed a number of frightening events which no one else did (or claimed they did not). All these events were to terrify her of a certain woman whom she did not know. One day that woman showed up. She was sitting on the couch and the main character rushed at her with a large knife and stabbed her. Luckily a coat button or something kept her from being seriously injured. The whole thing turned out to be a setup to get rid of both women by tricking the one into killing the other. It was very scary but the evil plot failed. Thanks to anyone who can identify this.