Hello, firstly so very sorry to hear you and your husband have separated. I of course don't know the ins and outs (and of course not asking), but just feel for you. Me and DH are hanging by a thread! Being a parent is so much harder than anyone ever tells you.
Anyway, re your DD, I wanted to comment (and hope this won't be a negative comment as not intended to be so, just realistic).
When you are a young child your parents are your world. Your pillars of strength holding up your world. When one parent leaves, even if it is just to another home, one of those pillars has suddenly come crashing down. It really is that black and white with children. They will express it in such different ways to you or I.
I don't say this for you or your husband to feel bad in any way as you are human and the black and white for us adults is that something's work and some don't. Plus you are both entitled to be happy, even if that is apart.
All you and your husband can do is constantly shower her with love and affection. That's all you can do.
My parents divorced when I was 6. My dad sat me down and said "I'm not gonna be living here any more but I will see you all the time and I love you very much". He ended saying "do you understand", to which I said something like "yea can I go and play now".
Well of course I didn't understand! That 6 year old girl was sat there subconsciously thinking "what on earth are you going on about dad? What do you mean you're not gonna be living here, where else would you be".
Asking to go and "play" I could make-believe that everything was ok.
For a personal reason later in life I had a course of therapy and all this became apparent.
To reiterate, none of this has been said to upset you or make you feel bad but at least with knowledge you can try to understand your DD a little better.
Wishing you all the best x