First, I think that it is wonderful that you felt you could come here and share your story. If just one person can learn something from it so they don't have to go through that sort of learning experience, then it's worthwhile.
I also think that it's great that you are are learning from your experiences - I have known too many people who never learn, they just blame the others and carry on making the same mistakes. Knowing what you want and what you don't want (the latter often being more important) is a very key thing, I believe.
Too often "secondaries" (and I use the term to mean someone who doesn't live together with a couple) get treated as "secondary" in the relationship, being used or treated like their feelings just plain don't matter. In my opinion that is just plain wrong and it does you a disservice.
Sometimes I hope that the "primary" couple are doing it out of ignorance, and that they, too, have learned by the experience, and won't make the same mistakes again, but I fear that some are doing it for altogether different motives than adding another loving relationship into their lives.
It's a pity that learning lessons like this has to be so painful sometimes.
"Listen, or your tongue will make you deaf." - Native American Proverb