Why is it so difficult to simply say "I'm sorry" and move on?
In any relationship it takes both parties to make it work or fail. Both are responsible. Whether it be poor communication, lack of honesty or maturity, both parties have an impact on the success and longevity. I think, IMHO, that any problem can be resolved by pulling together.
And, when people from a failed relationship reunite, for whatever reason, rather than anger and blame there needs to be a concerted effort to talk through past issues quickly and come to an understanding that both parties have, hopefully, changed for the better.
With a nice long talk and patience a lost relationship can be restored.