I would say to her (no e-mail, do this face-to-face):
"Look, sure, I've been enjoying some of the time we've been spending together, but let's be real here: less than a month ago, you basically 'fired' me as your husband, and just wanted to be friends. I'm not sure how I feel about you right now, and I think it would be best for both of us to take these things slow. Nothing needs to be decided today, right?"
She's looking for you to make her feel good about HER choice to break up your marriage, even though that's not what you wanted. I'm sorry, but that's not your job.
Be LOVING about this, tho -- not a dikk or anything. It sounds like you both genuinely HAVE enjoyed spending time with her, and ARE confused about things . . . right? If so, then my suggested "script" would be genuine.
When in doubt, I'm a big "speak the truth, in love" guy.
