I think you need to separate the marriage, your husband and the reason for your pain and betrayal, from the actual human being you are dealing with.
No matter how common these situations and affairs are, the basics are, that you are dealing with someone with a lack of concience, that goes too for his ap. To these sorts of people it may just be a game, a change of scenery or a feeling of one upmanship with a primary partner.
One thing is with a marriage you tend not to judge the person as you would an unknown or single separate person, for instance if you were younger, a teen, would you allow a friend to betray you so badly, to be ignored, to have them ally with another friend to pull you down, to actively harm your piece of mind, your confidence, your belief systems.
Marriage clouds the waters of undertanding, take away all the marriage expectations, sunkan cost fallacies and contracts away, you are left with the basics that the person who you share a space with is not a very nice person, and some the friends they make are also not very nice.
Start building on that, you never harmed your union or your friendship with him, you are allowed to judge youself and say that you were a nicer person with a different set of values, unfortunately there are many people out there that will not live up to your standards even that sole person who you share a confined space with.
Don't think of him as a husband, think of him as a human being, one who has greater flaws than yourself, recognise your strength in this, you have to separate the situation from the bottom line of how nice some people are and how some are not so nice.
It will take time to stand alone, mentally, you are undoing what has been your norm for many years, your friendship, your union, your ally and partner who you thought would protect you and be on your side, it's a lot to lose but you will build back up, time will make sure of that but you will become less trusting and garded with others, those feeling are there to protect you.
It will get better, and you never know what you will happen on this journey of discovering yourself, maybe you will find out that you are just too nice to be with this type of person who you put your trust in and gave away a big slice of your life to.
Get stronger and make the decisions when you are ready, not when someone who is a weaker specimen asks.
And you are allowed to support other people who have been harmed by bad people, it is your choice if you wish to befriend her h, but your loyalty now is to yourself and your children, show them the right way of being kind, the bad ones usually get discovered in life so their gains can be transient.
This is his shame, do not carry that weight off him, you did nothing wrong, you did the right thing, he didn't, one day you will pity him.