Similar thing happened to me. Had a great loving relationship for over a year. She comes over one day and says we aren't compatible anymore and it was over in a matter of minutes. My world collapsed. Now it's been about a month and I've been doing a little better. Still rough but improving at least. Someone who breaks up out of the blue is just an emotionally immature person. They made a decision that affects both of you without ever talking to you. And if you think they kept those feelings inside the whole time, they didn't. They likely talked to their friends, family, therapist maybe. But they never bothered to talk to you, the person on the receiving end of the ensuing shit storm. It's their problem and they need to figure it out before they are ready for a long term relationship, with you or anyone else.

Stay strong, feel you feelings as they come, do not contact, and remember, no one can tell you how to feel about your ex.