is this person "good enough" for me to spend all my time in a relationship with them, or should I hold out and wait for someone better?

I struggle with the same question a lot. People seem to different on their acceptance that relationships they attempt might not last, and that they might get rejected, or that their partners might find someone who is a better match. These attitudes aren't inherently related to monogamy and polyamory, but polyamory is probably more consistent with the recognition of the probable transience of most relationships.