How do I break it to this girl that I am back with my ex?

Honesty to me has ALWAYS been the best policy. Even tho is going to hurt, it will hurt now or later. The longer you put it off, the more it's going to hurt her. Be honest with her, tell her you got back with your ex, you do NOT want to hurt her, but above ALL in all fairness to her, you feel you must tell her the truth. Tell her you want to be the first one to tell her as you don't want her to hear it from anyone else which would only hurt her more. Wish her the best & tell her she'll meet the rite one for her in the end which will all be for the best. You think a lot of her, that's why you want to tell her yourself even tho you know it will hurt her. You're sorry, but must be true to her in all fairness...best to you...:)