Viper. First my sympathy towards you; breaking off of a long term relationship is almost always devastating. But now that I've said this (which I absolutely meant), I have some thoughts which might be hard to read. On rare occaisions, when a partner is feeling dissatisfied in a relationship, there might not be any real hints of that dissatisfication. This is most common when that person exhibits some degree of sociopathic tendencies--somebody who can be charming and decietful while feeling little to no empathy towards those who they are dealing with. I'm not so sure that this fits your ex-girfriend, from your description, although it might.
More frequently, a partner might feel dissasfied in a relationship, and thus be looking elsewhere, while only letting out the most subtle of signals. This is often due out of a desire not to hurt their partner and avoid any sort of confrontation from that disclosure. Unfortunately, this tactic towards interpersonal tension resolution only masks the problem, often leading to a more drastic unhappy resolution, such as ending the relationship.
This is where you, their partner, comes in. If your girlfriend wasn't always open and quick to share her likes and dislikes, you needed to be aware that those feelings still existed within her, despite her guarded candor. You needed to be both more probative, in an empathic manner, as well as be more open and sharing of your own feelings with her, so that she could feel more at ease with sharing her thoughts which she might be afraid of sharing with you. Yes that's quite tough to pull off, but it's often necessary to maintain a healthy long term relationship. If you talk with any truly happy long term couple about it, they will tell you that maintaining a relational atmsophere where each partner can share their fears and desires is crucial to the survival of that happy relationship.
I often point friends who have gone through what you're going through to read about Altman and Taylor's Social Penetration Theory, to gain some insights into how disclosure and our comfort in disclosing our deep feelings works to both make a relationship as well as make us pine for something beyond the relationship which were are in. That might be hard for you to do right now, as you're likely in a bad state, but after a bit, do a bit of reading about their theory (there's a nice discussion of it for laymen in Em Griffin's A First Look at Communication Theory) will likely provide you with some good personal insights. Beyond that, I strongly doubt that anything between the sheets led your ex to leave you for another--especially since you were able to provide her with erotic pleasure--but almost always the personal dynamics outside of the bedroom define the dynamics within the bedroom.
All of that said, every romance, and especially every break-up, can lead you to even greater happiness in your next relationship, if you learn from what worked and what didn't in the preceeding one. My best to you with that thought.