However, once it started, it was nearly completely a physiological problem.

Um, that sort of claim was studied extensively several decades ago. Having 'physiological' symptoms rather than 'psychological' did not affect the chance that a patient would have had stressful experiences, the chances of responding to therapy, or the nature of the successful therapies.

Exogenous and endogenous depression are not distinguishable in their clinical properties, which is why the categories ceased being used - they weren't useful.