Ten Lords a-leaping? Huh? When has that ever happened? Whatever....
"It's not you, it's me."
To be fair, sometimes the person actually means it when they say this. I'll say two times out of ten this is said with truth in the statement. (The number rises significantly when they you in question is ME but I digress)
Most of the time...guess what? It is YOU.
You did something that the other person couldn't deal with. I mean we are all screwed up to a point. Some people are worse than the norm though. This little gem of a statement is reserved for them.
Let's say that you are trapped in a cage with a wild animal who can snap at any moment. The only way out for you is to appease them and slowly back out of that cage. So you speak gently and walk back to the doorway while you keep a steady eye on the beast.
This is the same method you employ with the crazy person you are trying to leave because, just like a wild animal, they won't be reasoned with and won't change their nature because you try and show it to them. The best and only response is to say that statement, get your shit and bounce.
The problem I have with this statement is the ubiquity of it. People use this line so much that is a cliche and means nothing at this point. On top of that, most people can and need a little constructive criticism when their relationship is being dissolved. You don't just do this to make yourself feel better (read-feel superior) but you do this to help that person learn not to make those same mistakes in their future relationships.
I just re-read Things I've Learned from Women Who Dumped Me and, although a funny book written by comedians, the wisdom that these men gained from failed relationships is invaluable. It is indeed true that you can learn from the mistakes of others and this book is like a guidebook of how not to do stupid little things or how not to deal with women who will just use you with no hope for a future. I suggest both sexes check it out.
So, yeah, sometimes it is your fault and you should admit it.
And, yeah, sometimes you are dealing with someone who is so screwed up that you can't help him/her so say it to escape relatively unscathed.
But most of the time it's a joint failing and you should take responsibility for your own errors but don't let that other person off the hook. It doesn't help you and it definitely doesn't help the other person if you lie to them. Honesty is one of the pillars of a good relationship and a good break-up.
Because you know what? It is YOU. Cowboy up and change.