I agree with everyone about it becoming addictive. Hurting yourself is not a "normal" reaction to dealing with what is going on. There is a reason why you are reacting in this way. I strongly believe that cutting is not usually a choice but rather a reaction and it becomes an addictive reaction b/c in the moment you believe you "dealing" with the issue.

I don't mean to sound clinical but your wife does have reason to be worried, besides that fact that you are SI'ing, you might want to consider the idea that you are not dealing with what is going on in communicative way.

I know it might be a big step but talking to a therapist might be helpful. You don't have to make a major commitment, just one step at a time.

Could you put yourself in your wife's place? What would you think of her cutting and what would you do to help her? What would you say? Would you be concerned?

Just some random thoughts that I hope are helpful.