How do I deal with a difficult boss?

If I were you, I'd respond graciously and courteously to her email, thanking her for being honest and for the constructive criticism. Then I would offer her some positive things you have done recently, challenges you've accepted or overcome, progress in any area you have made and goals you have to improve your performance. I think she would be impressed with that and you would also have something doented to "cya" in when the review comes. Listing things you are working on , successes you've had and positive changes you want to make will all look good on your record. Keep the email professional, to the point and keep a copy in your files. Good luck to you.