What to say...
You love her, there's no doubt about that. I don't think the advice you seek is whether you should file for divorce or not--you've already made up your mind. And so, I conclude that you're here looking for a way cope. I can only hope to maybe explain some of the events that took place, and maybe that will help. Bear in mind that everything is my own take as a third-party observer who's only seen your side and your perceptions.
She loves you--despite her actions, from what you say, I believe this to be true.
Why would she cheat on you with this being true? I believe that, as you say, she went along with the possibility of better future, and has since, as you say, learned this was a horrible misconception. My husband and I have had several conversations about the possibility of a cheating scenario.. I've learned that my admittance that I won't cheat on him really won't assuage his worries; however, my admittance that I can't possibly imagine anybody I would rather be with will.
Why would she not tell you she was having problems? Fear of hurting you, ironically. I have already said that I am confident there is nobody I would rather be with than my husband... however, because I love him and I wouldn't want to hurt him our marriage, it would take a lot of will power for me to force myself to admit a problem with it to him. I would imagine that it's something I could work through on my own in time, and he (in theory,) would be none the wiser and when I finally do work through whatever problem on my own, everything would be happy. Is this how it should be? No, not at all; but I know that this is how my brain/emotions work, and likely other female's too.
Why would her friends not tell you? A combination of the same reason just stated and the idea that, it is your problem, afterall.
Why would they not persuade her to talk to you about it? Probably a combination of "misery loves company," and the female notion mentioned earlier--that things will go away, and he doesn't need to know about it.
Why would she continue to lie after you've plainly said you know the truth? Guilt... along with a glimmer of hope that you'll believe her. Again, the idea that it will be easier on you and the marriage if you don't know everything.
So...
Who is there to blame? Ideally, I think it would be easiest to get through it to not blame anybody and just accept it as "matter of fact" and work on what can be fixed--if you are religious, The Serenity Prayer comes to mind, and if not, there is still wisdom in it.
I know I've left gaps, which isn't my intention, but inevitable. Hope I've helped in some way, though.
If you wish, feel free to message me.
