Nothing. I think it's in the months and years after, when you finally get a chance to reflect on what went wrong and why, without the emotion, that you really learn anything.
Never had a bad romantic relationship. Had a couple of traumatising parenting relationships, from which I learned NEVER to act as a foster parent/guardian of a teenager you're not related to unless you have one hell of a lot of support (step-parenting is an entirely different kind of relationship). Teens are tough enough to raise if they're yours or your partner's.