Lovely Lily

Bill,
As someone who was in a similar situation a year ago, I urge you to make an appointment with a lawyer who works in both divorce law and immigration. He or she can help you bide more time with USCIS to work on the marriage or figure out the best course of action. You need to decide soon whether or not you want to stay and invest in this marriage, and if the answer is no, you should withdraw your Affadavit of Support. It sounds from some of your previous posts that you feel obligated towards your wife because of the amount of time you've known her. That's an honorable sentiment, but you can still choose to help her (short of staying married to her just so she can have a green card) without being legally mandated to do so. If you decide you don't want to be in the marriage, you shouldn't stay married, regardless of the effect on immigration. And the I-864 puts the USC in a very vulnerable position--I would not advise leaving that on the table unless you know you are in a healthy marriage with a committed spouse.

sometimes a guy just falls in love with a girl for who she appears to be. sometimes "taking advantage" and this kind of stuff is not even in the picture. sometimes it's just part of the background that there is a difference in economic status. sometimes the guy is the "naive" one in the picture. sometimes the guy is in it for the mind and soul the girl shares with him. sometimes she doesn't share the truth, but he has from the beginning.

Depending on your state you may be able to get an annulment instead of a divorce if the qualifications exist. Fraud is usually one of them.

Please follow Vanessa and Tony's advice. It is the best thing you can do.
While it is better that you find out before she receives her green card that she was not serious about the relationship than after, still it must hurt a lot. I am so sorry that you are having to go through this. You can certainly make an Infopass appointment for the USCIS officer near you and present them with your information, but you won't be able to attend the interview at that office. You would only be able to do that in the scheduled USCIS office. Going in person is certainly one way to let USCIS know but I would also notify them in writing that you are officially withdrawing the Affidavit of Support based upon desertion of your wife from the marriage.
The circumstances about the relationship are no one's business but your own and have no bearing on the best advice to offer here. You do not need to go into a discussion of them here even if people ask. Try not to be alone in her presence until after the AOS is denied for lack of a sponsor and you have had a chance to advise USCIS of her actions. She may not realize now that she needs you to remain in the US legally but she might very well learn that soon and try to come back long enough to get the green card or to set up circumstances where she could file for a VAWA claim.
Again, I am so very sorry this has happened to you.
Edited to add, yes, you can get a divorce. Talk with a family lawyer about the best means to do so, especially if you don't know where she is.
Yes, you will be able to file for another fiancee or spouse in time. You will need to provide all of the information about this situation, so do keep all of the paperwork for now as you will need to refer to it if you do find someone else in the future.

You cannot withdraw the AOS application but you CAN and SHOULD withdraw your I-864 asap. This will cancel the AOS application (as it's not possible for the AOS to go through without it). Go to whatever office you can get to ASAP and withdraw your I-864 in WRITING. Also include a letter and any evidence you have of her affair and leaving you.
If she attends alone she is denied. He HAS to attend in order for it to be approved.
I would be VERY careful that she doesn't try and claim abuse and get a GC that way. For that reason I would report to ICE that she left you for someone else and that you believe you were being used for a GC. NEVER be alone with her.

the OP is posting for advice not a scolding as if you know him and you're absolutely sure about your answers and you're sure the mother-in-law was working? how about a little respect

She should answer all questions honestly, without volunteering extra information. If she is asked the purpose of her visit, she can merely say "pleasure" or "tourism". If asked whether she has friends in the US, she should say yes. If asked if she has a boyfriend here, she should say yes.