Your husband said months ago, "its just one month. then it will be over".
It's not over, is it?
Are you content to live with this continuing, for the rest of your life?
Sounds like this is something you need to decide.
Why is it more important to him, to "not hurt her", yet continue to hurt you? You should ask him this, i'd say. Also mention that hiding his relationship with her, does not avoid hurting you. It hurts you worse than it already is. Hiding it, makes it more blatantly like an affair, yeah?
My current status: june 2006. Wife ran out and filed D. Finalized Jan 11, 2010, after 12.5 years M. 3 wonderful sons caught in the middle