Having just gone through this same thing, I learned the following:
1) Have a mantra you can say to yourself. Put together all of the best intentions that you have into a statement, and write it down. Read it out loud to yourself, record it on your cell phone and listen to it on your headphones, and use it to remind yourself of your true strength, desire for compersion and happiness for your partner.
2) Busy is good. Busy with friends is better. I had a number of friends on standby to talk me through my tougher times, and even had one of them stay over one night.
3) Go for a long walk or run, out in nature. It will clear your head and make you feel better. Exercise like a fiend, it gives you focus, brings you back into your body and helps centre you. Yoga and meditation are good too.
4) Journal, blog, or talk it out to yourself.
5) If you can afford it, schedule a counsellor or life coach session to get ready, and debrief.
6) Don't stay in touch via txt if you're struggling. I did that, and sent a few txts that I was none too fuckin' proud of, and have resolve to never do that again in the future.
7) Have movies that you like to watch, a book that you like to read, a football game to go to... whatever you do to feel good..... DO IT. Have LOTS of things that make you happy that you can do when you're feeling your demons come in.
Be gentle with yourself. This is a big bite to take for you. Not for her. For you. It was too big of a bite for me, and it taught me to be more realistic about the time my partner took with another partner - we're going to baby step it next time, as it was WAY too long for me.
You're courageous, strong and brave. You can do this. But take care of yourself, and be armed with all of the magic that you need to fight your demons the best you can. And you know what? It's okay to have negative feelings - they're feelings too. You don't have to tamp them all down, but if you want to talk to her about them, make sure you're clear about what you need first.
Hugs! And good luck! You can PM me if you want me to give you some backup support during that time, btw - been through it!