Be honest and answer any questions she has. Do NOT denigrate your ex.
You can't. She will have hope for years no matter how you explain it. Be honest but, more importantly, don't bad mouth him to her.
That was the hard part with my wife and our two older kids. Keeping her mouth shut about the things he would do and still would do so they could adore him. When I got in, I told them and her that they NEED a relationship with their dad.
Meanwhile he would cry and scream that I am not their real dad and that we are keeping him away and that he is their only father, not me. Keep in mind.. he wouldn't actually do anything to see them, just complain.
THat is a tuff one, just be honest. I am with my kids.
Weeee are never ever ever getting back togetherrrr! You go talk to your friends, talk to my friends, talk to me....:P
Make sure you explain it's not her fault and that you both love her (he is the father right?) then say that you just are better people and can be happier apart. Don't sugar coat it too much (risk her thinking you lie), but don't be absolutely blunt (risk of hurting her)
You're an adult, you'll figure it out