Be honest and answer any questions she has. Do NOT denigrate your ex.
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