How do you do that without a) being judgmental or bitter ("he made a 'bad' choice) or b) painting him as weak or deficient to his D ("he wasn't capable of doing the hard part")?
Well, if you give her information, you realize that 1) you're pulling her into your R with her dad; and 2) she's not mature enough to discern your truth from his and it will only be tougher for her.
I know it's hard but just say, "Daddy and I had some problems that we just couldn't work out. That's all you need to know right now, okay?"
When she's older, the game changes. My almost D22 has had a steady BF for 3 years. My R with her dad has been something that is periodically brought up more out of her wondering if things that happen in her life are normal? I'm a lot better at being able to tell her the truth about things. And BTW, after 13 years of being apart, I'm much more willing to tell her what I did wrong that hurt her dad too. It's easier to see myself back then differently - very similar to how we see our 13 year old selves?
"There are only 2 ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle."