I think with things like addictions, previous undeclared offspring etc, you are generally meant to forgive. Its the "for worse" part of the vow "for better or for worse." In fact, if I remember rightly, if you follow the Bible, Jesus said you ought to forgive 70x7 times, which I think just means forgive everytime. Whether to forgive means to remain in the marriage or not is another thing. Shan, whats your take?
Although you might be expected to forgive sometimes it just isn't possible. In order to forgive someone, the act/event you are forgiving must already have taken place. In the case of addiction, if the person addicted cannot admit to it then you are not in a position to forgive them, your only choice is to overlook the problems associated with it while trying to show them they are an addict. I couldn't blame a person for divorcing someone if they were continually hurting them/the family at the cost of their addiction. There is only so much someone can take and sometimes addictions are too strong.