I'm sorry to hear that in a childish and petulant fit, your ex decided he no longer wants to participate in the lives of his children. He really has shown his true colors here, hasn't he?
Please believe me and the others here when we say that this IS NOT your fault. He made a decision out of anger and spite that he will regret for a long time--you had no control over that.
I can only begin to imagine, though, the pain you must feel because of this. You and your precious children are in my prayers. Please come and vent and talk it out here as much as you need. Sending hugs!