I thought Tina B's post was both compassionate and insightful, but I also thought that Sometimes Steffi's comment that "I also had my CDing discovered rather than disclosed," was very telling.
When you hide the whole process, and then get caught in the act, of COURSE your wife flips out. At the very least she feels that she's been lied to and betrayed -- and from your post, it sounds like that's happened several times in the same way. If you make it clear to her that you don't want to hurt her, but that this is a compulsion that you simply cannot deny and that you WILL do again, she may still react with hostility, but at least she won't see you as a liar. I also strongly agree with those who have recommended joint counseling; just be sure you choose a therapist who is aware of and sensitive to these issues. Best wishes to both of you.