Figure Out What Went Wrong
Not all relationships end for the same reason. Women have an endless supply of unique reasons for ending things, many of which are often more complicated than they may seem. Your chief task post-breakup then, is to understand exactly why it happened. Chances are, if falls into one of two categories:
1. You messed up.
Admit it, you haven't been treating her right. You've been neglecting her, either subtly (by not listening to her, making her feel appreciated, or spending enough time with her) or blatantly (by lying, cheating, or otherwise disrespecting her). You may have thought you were getting away with it, but you weren't. The things she shrugs off day to day have built to a crescendo and she's decided she deserves to be treated better. (Side note: Good for her.)
2. She's doubting her feelings and/or doesn't love you anymore.
This is less about the mistakes you've made and more about her personal satisfaction within the relationship. She may have decided that the two of you are not emotionally and/or intellectually compatible, or that she cannot imagine a future with you. Feelings are not stagnant. Her outlook on you and the relationship is perpetually in flux. Falling out of love is a sad reality that can rarely be redressed.