Well it may not seem like any type of comfort but I'm very sure you can now stop referring to her as "The One" in your mind and to others, because if she was, you would be her "The One" too and she wouldn't be with someone else right now.

See someone can only ever truly be The One, if you are afforded the same label.

I'm really sorry you are hurting like this, please take the time to look after yourself CAREFULLY, like you would a dear friend. Do nice things just for you, spend time getting to know the you minus the girl. Healing will start once you're ready to embrace it.