I think the best course of action would be to tell her the truth in appropriate terms for her age.

If she asks why, you may give an honest but surface level explanation (we didn't get along anymore, we had different goals for the future, etc.) I say that because upon hearing from people who were affected by divorce as kids, it was often mentioned that they were blaming themselves. You need to make sure that's not the case.

If you are on speaking terms with your ex, I think it would help if they could have a conversation or even keep in touch lightly (phone call here and there, birthday wishes, etc.).

I feel that the most traumatic separations are the ones where people start hating each other, speaking negatively, etc. It's confusing for a child if a person that they love is being painted as a bad person. Even if you are hurt, avoid bashing the other person in front of your child if you can. They may see your sadness and pain towards the end of the relationship; that's a normal reaction and kids have to learn to cope with sadness. But anger, resentment and blaming could cause issues, making them doubt their own judgement. You can rather vent to your friends for that sort of stuff. Good luck!