Yes to counsellor, and yes to 'gethering evidence' as I understand that you need to know what is/was going on. But... how about you also try to move past all this him-focused stuff and think about what YOU, truly, want??
Because he isn't 'the love of your life', is he?
True love doesn't treat someone the shameful way he's treated you... and his children.
I couldn't forgive what he did and I wouldn't want to, OW or not. He treated you like dirt. I am not at all surprised that you are low in spirits... because deep down you KNOW that this is all wrong, that as others have said, he's treated you like crap, clicked his fingers and you've gone back. I think that you feel uncomfortable at some level and know that this has changed the power balance. That you feel taken for granted, and not valued. That you've sold yourself and the children short, and been left in a very weak position despite the fact that you are back 'together' - it's all hollow now.
And you'd be right - this horror doesn't value you and his family at all, and now he's proving it with the way he's continuing to treat you.
How did you eventually 'make friends'? - did he thaw towards you? Did it come from his side? Because if so, what I think is OVERWHELMINGLY likely to be the case is that he had an affair, left you to be with her, and it didn't work out. So he thought he'd have you back as that's better than being on his own.
I'll say it again - true love, and even just decent people don't treat their partners like that. And that he devastated your childrens' lives too - words fail, really.
Again - what do you want, really? Can you really say that you still love and admire this man? Can you trust his love?
I can only imagine the heartache and insecurity you went through and can totally understand why you went back. But I have a feeling that now the dust has settled, this sadness isn't going to go away. It's there for a reason, OP.
If I were you, I would go for counselling, and I would allow myself to think through a life without him. From where I'm standing it looks like it would have much to recommend it. And he could have the pleasure of getting what he once wished for, and trashed you and his family to take.
He's worth NOTHING, I assure you. You could do so much better with your life.