"how can lthey do this to me/ us" is one of the questions BSs ask - i did - and, for me it was partly my problem and partly my WS wife's - both problems stem from a realisation that what our minds wanted to believe was not the fact - In my mind I wanted a secure and faithful long term marriage (not happening so disbelief and suffering for me) - In my wife's case she was using her affair to escape from the reality of her own unhappiness at the lack of love from her mother and emotional abuse from her father, so she created a special space where she could believe her lover was her "special friend" who "really cared" for her (when the reality was that he was an arrogant and selfish man getting sex from a much younger woman who was also one of his staff). Her reality in her affair was also not in line with the facts - and she still finds the truth of the situation hard to swallow even now. The truth was that the man she selected for her affair was actually very like her emotionally abusive father.
So, you see, our sense of the world is not always in tune with the facts/ reality.
And the suffering you feel and, we have to concede, our WSs feel, is all about the pain coming from the difference between what we want to hold in our minds and what is actually real. And our resistance in facing the reality is where the suffering comes from. In theory this would mean that if we really accepted all the truth of our world we might be happier - but I admit to still struggling to accept the world as it is - I still cling on to the ideal view of the world I want to be there, including a perfect soul-mate loving partner etc etc.. I know these are probably impossible dreams and yet I still seem to be able to conjure up the ideas that this is how the world is in my mind.
Reality is tough - but the truth is the safe place to be happy. Facing this is what being alive is all about for me - I just hope I get to be more fully alive whilst I have the chance.
may you be safe and well, contented and happy