I took the meaning to be that he is able to compartmentalize the bad things that he and others have done. His forgetting would be part of a coping strategy that he had put in place prior to everything happening. If he already had a coping strategy in place to keep stress down, then as things progressed, he would use "Motivated Forgetting" to keep his sanity and be able to deal with his wife being with Negan, the iron to his face, etc.
From the Wikipedia: "Motivated forgetting is a theorized psychological behavior in which people may forget unwanted memories, either consciously or unconsciously. It is an example of defence mechanism, since these are unconscious or conscious coping techniques used to reduce anxiety arising from unacceptable or potentially harmful impulses thus it can be a defense mechanism in some ways. Defense mechanisms are not to be confused with conscious coping strategies.
Thought suppression is a method in which people protect themselves by blocking the recall of these anxiety-arousing memories. For example, if something reminds a person of an unpleasant event, his or her mind may steer towards unrelated topics. This could induce forgetting without being generated by an intention to forget, making it a motivated action. There are two main classes of motivated forgetting: psychological repression is an unconscious act, while thought suppression a conscious form of excluding thoughts and memories from awareness.
Roland Benoit, a scientist at the Medical Research Council, Cognition and Brain Sciences Unit at University of Cambridge states: "... we use two different ways to deal with bad things -- suppression or substitution -- to avoid thinking of uncomfortable or unhappy memories."
While both processes cause forgetting, a different region of the brain controls each one. When people suppress memories, the dorsal prefrontal cortex inhibits activation in the hippocampus, which plays an important role in retaining memories.
“It thus effectively breaks the remembering process. This, in turn, disrupts the memory representations that would be necessary for recalling the unwanted memory later on,” Benoit explains.