But it isn't crap. It's perfectly reasonable to hate the OW's behaviour, because it is......hateful. Wouldn't be rational to hate her more of course, but sleeping with a married man is behaviour worthy of anyone's blame, but especially by that man's wife and especially right now.
It's fair enough to hate the pair of them for now, but what might help you is to separate the behaviour from the people themselves. You know your husband and although you've described him as a selfish arse before the affair (now why doesn't that surprise?) you also know the good bits of him. You are also seeing what appears to be sorrow for his actions (although at this stage, a lot of that will be sorrow for being caught) whereas the OW is a one-dimensional pantomime villain whose sorrow or regret for the hurt caused, is unknown.
Don't give the OW too much power by making her something she's not. She's just an ordinary person who like your husband, made some very bad choices. She lacks the skills to do what you do, day in and day out with your son. If she's a decent person in the main, one day she might regret the pain she jointly caused and be sorry. But like you at the moment, she probably can't feel that magnamimous and is focusing on her loss.
One day she will have to confront the fact that if your husband had agreed to leave for her, she would have been complicit in the break-up of a family. Some OW never get past the stage of hating the MM or the woman he has chosen to be with and that is actually their misfortune, because they are losing out on the opportunity to examine their own destructive behaviours. Whether she feels regret, loss or decides to blame everyone else but herself, one day you might feel sorry for her and hope that she will learn from it, just as your husband appears to want to learn from his own hateful behaviour.
Right now though, give vent to what you feel, because it seems to me that your anger has been slow to come through - and that isn't necessarily a good thing.