My way of thinking, while unpopular, is still my way of thinking on this.
In my opinion breaking up with someone over the phone, IM, email, etc. is the chicken's way out (not that you are a chicken by any means). The one being broken up with may not GET IT. It is easier to ignore the facts even though they are staring them in the face in full fledged black and white letters.
If someone wants to break up with me I would much rather have them do it in person, face to face because then I can gauge how serious they are by their body language, tone of voice, etc. It's also easier to say good-bye in person, at least for me.
I've done one break up via snail mail & I still feel guilty about it even though it was 20 years ago and one over the phone. I didn't have much of a choice in either case since #1 was in jail & I couldn't mentally face the five hour drive to visit him in prison AND the break up itself. The other one was an LDR and I figured phone was still better than email or IM.
If there is any possible way you can do it in person you should. You will both probably feel better about it afterward and you can then make a clean break without the hassle of the back and forth emails about why you want to break up with her, etc.
As to what you should say....I think you already know what you want to say way down deep
. As for me: "Listen, I know we've talked about this a lot but I've come to some realizations after doing some reading and research and come to the conclusion that this relationship is toxic for me. I need to go and get myself healthy before I can consider ANY relationships with anyone in the future. Good-bye and good luck."
After the conversation (or however you decide to do it) don't open any of her emails, answer any of her phone calls (or those from her b/f if she tries to go that route). If you have to you can block her email/IM and you can block phone numbers for a minimal fee with your cell or landline provider.
Good luck. I can see you will go far if you decide to stay in this lifestyle just make sure you take some time out afterwards for introspection and grieving and to get your mental state healthy again.