I met my ex on a dating app in 2020. I was ready to settle down, and he seemed to be too, so things got serious quickly. We bought a dog and picked out an engagement ring. Then, it all fell apart when I learned he had three other girlfriends, all of whom thought they were exclusive with him. I have a doctoral degree in public health, specializing in social and behavioral sciences. I'm a smart person, so I blamed myself for missing the signs — though even in retrospect, I couldn't see any. Almost immediately, I realized there were...