The email your Mum sent you must have broken your heart. But do you know what? I can see myself writing exactly the same to my son in a few years' time (he's still only 10) if I hadn't had my awakening only a few months ago. I understand her perfectly. How much she loves both Jehovah and you! She must have been a wonderful mother, and she still is. There's no doubt she loves you to bits. She can't help feeling that way. Most of us would have done something very similar before we discovered reality. And honestly, I don't think anything you say right now will awaken her. She's just not ready yet, if she ever is.
That's why if I were you I wouldn't even try to explain your reasons. I would just, as other posters have pointed out, reassure her of your love for her, no matter her reaction. Tell her you understand she may have to shun you in the near future, but you will still love her all the same. AKJeff said something beautiful that reached my heart a few days ago and it applies here: "Jehovah's witnesses have no antidote against love". Sticking to Watchtower policies about shunning will be extremely hard for her, and she should be admired for obeying the dictates of her conscience, even if we, from the outside, are horrified by that treatment.
That's all I can think of telling you at the moment, except wishing you lots of happiness with your Sarah.
By the way, I'm Spanish, from Madrid, where I've lived all my life. Feel free to PM me in that language if you feel comfortable (I've just read the topics you have started). I definitely express myself far better in my own language.
I wish you all the best.