I agree with a lot of PP about what you need to do here, OP, and I’m so sorry.
I’m also a little less nice than some other people– I think you are well within your rights to put them both through hell if you can. I believe that there is such a thing in this world as an enemy, and when people say it isn’t the other woman’s responsibility I have to truly disagree. (Basically I don’t think a woman owes it to a stranger to not try to steal her husband, BUT she should understand she’s choosing to be that wife’s enemy, actively choosing to do her harm, and it is perfectly appropriate for the wife to be extremely hostile.)
Here’s what I would do, in order, and preferably all on the same day so they don’t know what hit them:
-Call a moving company to come and pack up all of your stuff, all of your joint homegoods, and anything of “his” that’s particularly valuable, that you think you can sell, or that you like. You’ll end up giving some of it back but no reason to be nice about it; neither of you has any of the legal property rights guarunteed by a marriage, so it will be a bit of a free for all and I’m sure you’ve heard that “possession is 9/10th’s of the law.”
-Let your landlord know that you’ll be breaking your lease; be prepared to pay any necessary penalties and not worry about it. (You may have to arrange the movers a few days in advance, plan for them to come on the day you want this to all go down.)
-Call your mom and best friend, briefly explain the situation, ask for their help cancelling the wedding, and for a complete list of all wedding expenses incurred by your family, and ask whichever one you prefer if you can stay at her place. Direct the moving company with the boxes of all your stuff to her house. Pack a suitcase to live out of with about a week’s worth of stuff.
-Get a private account and take all of the money. All of it. You will have to give some back later (consult with an attorney, and figure out how to give back the bare minimum). Start hiding it wherever you can. Again, he’ll get some back eventually, but meanwhile, let him squirm. (You may need to get your accounts secretly set up a couple days in advance and contact a lawyer in advance as well.) Leave him enough to briefly survive, or: his half, minus what he spent on her, minus all wedding related expenses, minus any fees for breaking your lease, minus any expenses you may expect to incur in the process of finding a new place to live, minus any expected fees by your lawyer, minus a little more because he should suffer.
-Contact your bank about closing or taking your name off of all joint accounts.
-Physically leave your home and go stay with whoever. Instruct your mom and BFF that you will not be taking any communication from Fiance and ask them both to respond to any communication by him that they aren’t interested in speaking with him and to please forward any communication to your lawyer.
-By the way, if Fiance wants any money or stuff back, make him take you to court. Have your lawyer available, as well as documented wedding expenses, and proof of the affair. Make it hard for him.
-Finally, and hopefully you’ve pulled this all off during a work day, drop the ball on your Fiance: Send an email to his boss, her boss, his boss’s boss, her boss’s boss, (the boss’s bosses are important because his and her immediate supervisors will be SO PISSED at them for making them look bad) and HR, along these lines: I’m writing to inform you of an inappropriate relationship between [FI’s name] and [affair partner’s name]. In addition to being colleagues, [names] are involved in a romantic relationship. Please see attached for evidence; [FI] has also confirmed this to me. You need to be aware of this issue, as it is in violation of your company policy, and because the legal disputes which may result from [FI’s] actions, as well as impending general knowledge of this situation, will likely affect your team and workplace environment.
-Let that email be the way Fiance finds out about your decision. Send him a follow-up email letting him know that you’ve claimed your assets, dissolved your lease, closed your joint accounts, and moved out of your joint home, and that you wish him all the best but do not wish to be contacted by him in the future. Ask him to direct any inquiries to your attorney [so-and-so].
-Then take some time to yourself and recover. Let people know, briefly but honestly, what has happened, and ask them for whatever support you need– be that a shoulder to cry on or (if it were me) just being left alone for a while.
-Feel better about the fact that with no home, no belongings, no money, not much clothing and no job, and an impending court battle with you to get favorite chair or w/e back, your fiance will soon have no girlfriend (if she has any sense in her head) and you have, therefore, sucessfully put an end to his affair, as per your request in your original post.
I’m so sorry this has happened to you, and maybe my revenge fantasy outlined above is too extreme for you, but I hope that you end this engagement, and get through this process, and make things hard on him!