Maybe it is that old adage at work: You learn more from your failures than your successes.
Hi. I'll venture forth with the embarrassing revelation that I'm in my mid-thirties, have been divorced twice, and am on marriage number three. I can't say that failure is a better instructor than success, or that success has less to teach, but that, for the willing and appropriately humbled, failure can teach you a lot of really important things.
Failed relationships hurt. They involve a lot of different personal and social dynamics. They give a lot of great examples of stress-test cases for how we react, feel, and behave toward ourselves and the people closest to us. There's some pride-swallowing involved, and by necessity one must play devil's advocate with a lot of core beliefs and assumptions. "What could I have done differently to make this work? What did I do that I regret doing, or that made things worse? What made me feel this way, and what made the other person feel/act the way they did?"
Personally, I learned that a lot of my beliefs about and understandings of what makes for healthy relationships, as formed in my childhood and teen years, was just plain wrong. I had to teach myself how to detach from a bad situation to look at it more objectively than I could in the heat of emotional response. I learned to start applying the same basic problem-solving processes and forward-thinking that I've picked up in my professional and business life to how I interact and communicate with my loved ones. Most importantly, I've better learned to live by acceptance and understanding rather than denial or inflexibility.
I'm not saying it takes failure to learn how to be healthy and in a healthy relationship - but if you weren't lucky enough to learn how to do it before ending up with problems, then reading about how others learned from their mistakes has a lot of potential to help.