@FirmSwan the best thing to do is to start your own thread so as not to derail this one.
Bear in mind that men are not always treated kindly on Mumsnet, with kindness, thus might not be the best place for you. There are forums on the Surviving Infidelity website and on the Affair Recovery website, both free of charge, where you will find men as well as women in your position. There is far more understanding and compassion there.
Having said that, just for now, as this is @Strawberrina ‘s thread:
1: no matter what the state of your marriage or your part in that, her decision to cheat is not your fault, absolutely not. Accept no blame, you are in no way to blame for that decision.
Your marriage is 50% your responsibility and you have to own your half, but her decision to cheat as a way to deal with your marriage issues was 100% her decision, she could and should have chosen an honest option. Blaming you is just guilt deflection and an unwillingness to own her responsibility for her dishonest choices. She is also 50% responsible for the marriage and after she has fully owned her decision to betray you, might want to look in her own back yard.
2: whilst she is disrespecting you like this there is no way to stay in this marriage, at present she is acting as if she’s not married at all. Until/ if she stops the affair and wants back in, you need to plan for your future.
3: Give yourself time to accept what is going on and adjust to the new reality. Trauma causes damage so protect and take care of yourself, you can only control your behaviour, not hers, so look after your own wellbeing.
Find boundaries to protect yourself, tell her you cannot tolerate her affair or disrespect of your marriage and decide what ultimatum you need to set and how to follow through.
This isn’t on you, this is her unhealthy way of selfishly staying with you and getting her kicks elsewhere. Don’t accept blame for that, she had options.
Sorry this has happened to you, it’s horrible.