As a card-carrying Lousy Judge of Character, I've been there many times. All you can do is shrug it off, say, "Whoops, I made a really bad mistake for a really long time," and go on. Once you've healed a bit, if you want to you can try to go back through the events that led up to the betrayal to see if you can identify patterns to look out for in the future. But in the end, people are unpredictable. We Earthlings have wildly divergent ideas about what "friend" means, and it's not uncommon to learn that way, way too late. You weren't stupid or in denial - it's just part of the human condition.

Love, Columbine