After much thought and prayer, I've decided that I cannot sign your letter. I simply can't pretend that our marriage is something that it clearly isn't, and I certainly won't lie on paper to cover up what you're doing here.
I hope you can find another way.
If you need to pick up any of your stuff, please let me know what would be a good time for you, and I'll arrange to have someone here to let you in and see you out. It needs to be by the end of the month, or I'll assume you don't want it, and will give it to charity.