I've been there many times, and it's horrible. Yes, I did the same thing myself but only after it being done to me for 25 years, and then he didn't even care. I'd like to offer you some support because I know how horrible it feels.
The first thing that goes through your mind is you are trying to find some way for it not to be true. Some way to make it not be true. Some way to change the other person's feelings or desires. It's hard to accept, but you can't do that. Whats done is done and what you have to do now is to work with what you have to salvage it, if you want to go that route.
The first thing I'd suggest is give yourself 48 hours before you make any decisions. Don't let yourself be sucked into the whole "I have to fix this right now!" trap. A sense of urgency is not a good thing because you can't think clearly when you are feeling that way. You have to give yourself some time to get over the initial shock and think straight again.
Keep in mind that while you are this upset, she's definately having some upset and unsure feelings herself, and I'm not saying that so you will feel bad for her, but so you will know that how she sees you react right now will definately have an effect on the outcome of this whole thing.
You want her to turn around, come back, feel very bad and guilty and cry, wish she hadn't done it, apologize, beg you not to leave her and promise to do anything to work things out. At least that's what I imagine you want, because it's what I always wanted my husband to do, which he never did. It feels like if you scream enough , or have enough of a fit, or break enough things, she will see how much she has hurt you and want to fix it, but that usually isn't so. You may think that doing those things will make you feel better, but it really didn't much help me, it just made him think I was a psycho b***h that he was better off without.
So, here is my advice, if you want it. Take a few deep breaths (literally, it does help to calm you down some) and then decide to stay level headed and appear calm and rational when you talk to her or have any contact with her. That is the number one thing to do. You staying calm, although cold, can make the other person go "uh oh, maybe this was a bad idea". If you freak out on her that will just give her a reason to go over to his place. You don't want to give her that.
If staying calm is number one, then number two is do not initiate contact with her. Let her contact you. She will. This can take a few days. Don't do the drunk dialing thing. If you do drink tonight, put a sticky note on your phone reminding yourself of this. It is not a good idea to contact her. The more silent you are, the more nervous she will get about whether or not she did the right thing.
As much as you might think its a good way to get back at her, don't go out tonight and get you some strange. That will just cause complications if you decide to stay together and it actually will even the score in her mind. There is nothing you can do right now to get her back for it. The best way to get her back for it is for her own guilt and doubt to eat at her, and for that to happen, you have to react in certain ways which are completely opposite to natural instinct.
It's never easy, and it's never pretty, but it is possible to salvage your marriage. She's going to have to come to you though. Don't sound needy, don't beg, don't do any of that. It's very hard not to, but don't do it.
I truly feel for you, I do. I've been there myself so many times and it's one of the worst hurts you can feel. Try to keep your head and try to stay calm. Don't do anything stupid or anything she can use in her mind as an excuse.
If you want to PM me, I'll be glad to talk to you.
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I'm giving it another shot. We will see.
My forum is still there and everyone is welcome to come join as well. There is a private women only subforum there if anyone is interested. Also, there is no CAPTCHA.
The link to the forum is
http://www.rightplanet.proboards.com