In this passage, all that the doctor is really saying is that this is a case where the patient (Lady Macbeth) has to heal (minister to) herself. The doctor himself can do nothing to help fix her problems. What is going on here, of course, is that Lady Macbeth is not physically sick. Instead, she is having mental or psychological problems that have been brought on by guilt.
As we have seen in the hand washing scene, Lady Macbeth feels very guilty about the things that she and her husband have done. In the line you cite, the doctor is pointing out that it is her guilt that is weighing on her and there is nothing he can do to help.
The doctor, here is giving an answer to Macbeth's question. Macbeth had asked him if was not capable of addressing a sick mind. Is he only capable of ministering physical sickness? Can he not come up with a calmative for the sick and sorrowful mind? Can he not uproot from the horrifying passages of a gulit-ridden memory, the cause of inner-grief, that seems to madden Lady Macbeth?
It is in the form of an answer to this question that the doctor says that in such cases of mental trouble, it is only self-ministering or self-conducting or to use a modern Freudan term, 'self-analysis' is the only way to go.
Most of the psychological problems can only be solved by the patient and this is a psychoanalytic truth spoken by the doctor in Shakespeare. The process does not lie in a removal of the symptom but rather an acknowledgment and emptying out of it.