What really matters is that you realize you did nothing wrong and that you couldn't have prevented this. In order to heal, you need to have a sense of closure. Why it ended needs to be clear to you; otherwise, you are likely to have doubts about the relationship and yourself for a long time. Let your ex-girlfriend know that an explanation, or one reason, for breaking up with you will help you to go on with your life, just as she has been able to go on with hers. She didn't do this without any reason at all. Even if it has nothing to do with you, you deserve to know at least part of the reason why. She still may not tell you. In fact, she may not even know, but at least you will have asked.
Although you need to grieve for this loss, make sure that it doesn't consume you. Talk with friends and family about how you feel. Knowing that people in your life love, support, and care about you can be comforting. Know that you are a good person, that you will be okay on your own, and that someday, you may meet that special someone who will feel the same way as you do about sharing your lives together.