You could always say that you need to have time to be away from her so you can miss her more! That'd be cute
Otherwise, unless you lie, all you can do is tell her the truth! Just be up front like everyone else has said...but maybe try and be a bit more subtle...
It might be hard for her to hear so be gentle. Tell her over and over again how much you love talking to her, how you still want to talk a lot! But that you just need a LITTLE more time to yourself... Keep reassuring her over and over that you care about her and that this isn't about her and that it doesn't mean anything is wrong in your relationship, but that you just need a titch more space.
Hope everything works out!