What does the Doctor mean when he says, “Therein the patient/Must minister to himself?” in Act V?
The doctor is merely stating to the nurse that Lady Macbeth obviously has a guilty conscience and that she has done some horrible things in her life. This is why she is sleepwalking, as her soul will not let her sleep soundly as a result of the evil she has committed. In order for him to be able to help her, she must first admit in her waking hours her guilt and seek help for what she has done and what she knows has been done--rather like someone with a drinking or drug problem--first you must admit there is a problem, then you can seek help which will actually do some good. Until then, nothing works.
The doctor is trying to explain why he cannot help or cure Lady Macbeth. He realizes that there are things within her conscience that have to be worked out, specifically her guilt, that medicine cannot fix.