Whatever she wasn't getting from you, she got from him. To women, it's probably emotional. Women, want and need love, communication, the feeling of being nurtured, and being wanted.
I've been guilty of negligence on all these counts, and my wife has let me know. She has spoken with other men. She has found in them the qualities she wishes I had, but also thankfully, seen the qualities they lack, and I possess.
They key is to meditate on the reasons you fell in love in the first place, and try to be a better person, each one putting the other person in first place.
Easier said than done. You both have to willing to concede. I hope she will come to her senses and come back, but if she doesn't YOU will grow from this painful experience.