Hi justlost. I joined after lurking here for a few years for essentially the same reason as you--trying to understand what's happening--except that our roles are reversed. I recently broached the subject with my wife of more than 30 years, not helpfully summarizing us as "non-practicing poly" (me) and "serial monogamist" (her), when in fact I'm mostly confused by my feelings and she is upset by (what I've managed to convey of) them, and by what I perceive to be the cause of them.
Not sure I can offer any insight but in my case the roots may go back to my adolescence & just snowball from there. That would have to be confirmed by a psychotherapist however, and things would have get more out of hand than they are for me to consider it.
AutumnalTone -- thanks for the guidance.