WF please do not have the affair. I can't see anything good that will come of it. I agree with the others here. If you truly are done with the relationship do it through the legal channels. I don't think you will be happy with yourself afterwards if you were to go through with it either. Just because he isn't acting the way a husband should, doesn't mean you should seek out something you will most likely regret later.
I always love to hear happy endings, so I really hope that your husband gets his stuff together & comes around. Please try to seek all other options first.
Good Luck!