I guess it was from a not-so-good poly resource. I can't tell which are good and which are not-so-good, so I've just read everything. Xeromag was a great help, though, and I should have started with that.
The resource I'm thinking of I recall repeating over and over that no relationship is permanent, that everything ends, and while that's true of many relationships, it's not something you want to keep in mind as you're in a relationship. And I'm a pessimist by nature--I didn't let myself acknowledge that I liked him until he told me he had feelings for me, and almost as soon as we became boyfriends, I started to try to prepare myself for when it ends. Then I realized that if I thought all the time about it ending and I didn't let myself live in the moment, I would never be happy in the relationship. So when I encountered such a pessimistic attitude, I was really worried.
I'm glad that the attitude you're expected to have going into polyamory is much healthier than that. I guess I had trouble seeing the difference between not caring and being able to move on.