Maybe you should take a break from romance for a while, until you gain a better understanding of yourself.

I can understand your feelings of guilt for not having deeper feelings for your ex. The guilt may be why you feel sad whenever you think about her ~ perhaps you've conditioned the emotion to everything related to your ex. It's possible. But, that' s for you to think about.

I would advise you to avoid contact with your ex, while you're working through your emotions. No point in hurting her or yourself any more. Take some steps back so you can see things more clearly. Talk with a professional if you need help (feeling lost, hopeless, etc.).