Good for you for going out. I used to feel awkward and uncomfortable too. It was NEVER what I wanted. But each time I went - it got easier. And I learned to accept things as they are.
I know that it is not what you want but you have absolutely no control over what your H does. You simply cannot react to everything he does. Or OW does. Because you seem to base everything in your future on what you know of the 2 of them. You should stop finding out what they're up to. It does not do you any good. It makes you feel terrible. I used to do it too. But when I stopped worrying about what they were up to - I started to live again. You know he lies. You know he cheats. It doesn't mean he will never come back. But it also doesn't mean he will.
Best advice I can give you for this stage in your life - go talk to the lawyer and get all the info you can in order to protect yourself. But - since you are not ready and don't want the divorce - don't file. Never threaten to do something you don't want to do. And don't DO what you don't want either. You may reach a stage where you DO want it. Do it then. Not before. Let him - if that's what he wants.
I wish I could take away your pain. I understand it so well. I lived through it all too and would have given anything for my heart to stop hurting. And to make everything right again. But you have no control right now except over your own behaviour. Doing your 180s. Taking care of your son. And trying to go about your own business.
Going out was a 180 for you. Your H will hear about it. It was a good thing in more than one way. GOOD FOR YOU!