They say that every relationship, whether successful or not, teaches you a lot about yourself. So what were the main lessons (good and bad) you learned from each failed dating relationship (assuming you've had a few that lasted from a couple of months to years). I'll start.
Guy #1: Got my drive to be an academic (and topic of my PhD) from being around him. Learned to publish my work ASAP. Also learned that fake people (him) are dangerous.
Guy #2: Academics are not necessarily to be respected. Learned to write and edit properly from him. Learned to not bother with men who are seeing another woman occasionally.
Guy #3: That it is not my fault he could not get it up. We could still be friends.
Guy #4: Don't get involved with a narcissist who says from the beginning he is not good at relationships. Became more athletic due to him
Guy #5: Neediness is unattractive. Men who want to be taken care of not for me.
Guy #6: Listen to instincts when they are telling you that a guy is crazy (he is schizophrenic apparently). Avoidant personalities come on strong initially.
Guy #7: Younger guys can be more mature sometimes; learned about spirituality and to love and work on myself more.
Now it's your turn. I should note that this probably works best when the poster is over 30 and has had some dating and life experience under his/her belt.