Short answers, pulled from the story.
Compartmentalization is a psychological defense mechanism in which conflicting thoughts, feelings, or cognitions are kept separated in the mind rather than confronted together. Its primary purpose is to avoid cognitive dissonance, the mental discomfort caused by holding contradictory values or beliefs simultaneously. It may be a form of mild dissociation.
Isolation separates a thought from the feeling that belongs to it, while compartmentalization separates different, incompatible cognitions from each other. Both are psychoanalytic concepts, but they operate on different aspects of mental life. Compartmentalization is also linked to rationalization and to the pattern psychoanalysts call neurotic typing.
Otto Kernberg used the term "bridging interventions" for a therapist's attempts to straddle and contain the contradictory, compartmentalized components of a patient's mind. The term refers to deliberate therapeutic techniques for spanning the mental gap the patient has maintained between incompatible self-states.
People with post-traumatic stress disorder compartmentalize positive and negative self-aspects more than those without PTSD. This helps prevent negative self-aspects from overtaking positive ones. For those who have experienced sexual trauma and been diagnosed with PTSD, compartmentalization can specifically protect a positive self-concept.
When compartmentalization functions as a major defense mechanism, it can create hidden vulnerabilities related to self-organization and self-esteem. If a negative self-aspect is activated, it can cause a drop in both self-esteem and mood. The stability compartmentalization provides depends on separation rather than integration, leaving it exposed when the mental partition breaks.
Doris Lessing identified the essential theme of The Golden Notebook as the argument that "we must not divide things off, must not compartmentalise." She listed the paired categories she meant, including "Bound. Free. Good. Bad. Yes. No. Capitalism. Socialism. Sex. Love," treating each binary as a temptation toward false partition that the novel resists.