This is so sad. I thought the last thread wasn't real - I'm sorry!
There's no rush - work out what you want to do. If it helps - you are still young!
You're asking why, why, why. It might just be that your husband is incapable of having a full, honest relationship. So it's nothing you did.
It sounds as if he sees you as strong and fulfilled, which he mostly wants, but he also wants to help save someone he sees as in need. That he has categorised you and the other woman in this way is very wrong in itself - and helps him justify his terrible behaviour to himself.
It's all rubbish of course, but the funny thing about unfaithful people is that they often think they are extremely moral, it's just that it's somehow a special and complicated situation in their case, so that they are off the hook. And it's the apparently nicer, kinder people who largely get away with it - others feel sorry for them.
Yes, his family and friends would/will back him. It's so sad, and unfair. But you don't need them.
You mentioned that he is 'religious'. It sounds as if there are many reasons for the religious marriage to be annulled, assuming it was a church or religious wedding.
If I were a parent of your daughter's friends, I would want to listen and help. You really do need the support of people outside of this situation. And you will build a wonderful life - with amazing new people who value you for who you are. There will be dozens of people you have met outside this group who already like and admire you, and would love to know you better.
You might find that once he gets a sense the marriage is over - and irreversibly - he will turn on a sixpence and he'll become your adversary. So be careful and make sure you can't be deemed to have left. He needs to be the one who leaves.
And - I'm so sorry that your mother has been useless. If I were your mother, I would say you should do the right thing for you - you need and deserve to be happy, and your children's happiness depends on you being happy. Don't think that you can accept a compromised life in the interests of protecting 'the family' or because you see other people as more deserving than you. Who would do the same for you? You need to fight for yourself and your own happiness. And you have the right to be happy.