I get why you are scared, and if you love him you want to be told it will be ok.
From the info you have given it sounds pretty bad, especially continuing after he told you it was over.
I don’t believe you are alone in this scenario and many people chose to stay, people just don’t want other people to know so nobody talks about it.
I don’t think you can trust him but I also think no one can tell you what to do, you will do what you want to and it might be a long process.
Your heart must be broken and you will want to cling on to hope, you aren’t a doormat, you are just a person who believes in love and marriage.
He has treated you very badly and I wonder how he has tried to reassure you?
For me, his excuse of her not allowing it to end is rubbish but I don’t think you can think clearly when it’s you and you love someone and your heart has been broken.
Be kind to yourself, focus on your kids, spend some time and money on yourself with your friends. Do some stuff without him, spoil yourself, you are entitled to after what he’s done. Some little pockets of time away from him, even pretending to be ok might give you some clarity and help you to make a decision.
You are more than just your marriage, just remind yourself of that and it might give you some strength to make decisions.
Don’t listen to anyone who tells you negative things about you, you haven’t done anything wrong, you have been the victim of a betrayal and it will take time. You aren’t a doormat, you were a wife who trusted her husband, people are always quick to judge when it’s not them it’s happening to.
its your life, so listen to yourself and your heart and do what you need to do to get through it, even if you chop and change your mind, you are allowed to.