Why can't I figure out the lesson here?

While you offer enough to get the measure of this man, there is little about your own life experience to join the dots and this is not the place to do that. Those who feel drawn to help others with their difficulty's help those who most reflect the difficulty's they themselves most need help with, so its no surprise he is a psychologist as his issues are very clear. People are people whatever profession they choose. Relationships are about inner growth and it seems likely this one brought to the fore an unresolved aspect of childhood where the desires of the child to find 'approval' overrode the good sense and reason of the adult which has raised serious questions for you. If you were unaware of that aspect of your being and continue to punish yourself with the sense of shame, perhaps you might consider a brief inner journey with a female professional to explore this. The silver lining of this dark cloud may be the restoration of inner harmony and an enhanced level of happiness.