The honeymoon period is short and sweet in every relationship. If your ex said she felt like you were just roommates, I think there was some underlying problem that caused her to feel this way. Maybe the two of you were not really compatible. Maybe you did not have what it takes together to build a long and lasting love. In this case it is better for you that she has gone. Or maybe her unhappiness was related to a personal problem in her own life that she needed to address. Or maybe here was a dynamic in the way the two of you interacted that caused her unhappiness.

I guess you will never know the answer because she did not stick around to try to work this out. But I am sure this is very painful for you and I am sorry you are going through this.