@DownNtOut,
Why did you break up in the first place? Who ended it? If it was you who ended it, then she may be scared that you'd do it to her again, and not want to get too emotionally involved with you. If she ended it, then it sounds as if she likes you a lot, but can't feel stronger emotion for you.
If she cared enough for you, then she wouldn't be with someone else. She obviously feels comfortable with you - sufficiently so that she slept with you, and enjoyed your company, but after going out with you for a year, that isn't unusual.