How are you feeling, littlerayofsunshine0? I'm hoping that as time passes you will be regaining your joy in life, with your sweet DC.
I'm sure many people love you (most of all the DC), but you don't seem to love yourself as you deserve. And I suspect your ex was quietly chipping away at your self-esteem for many years before he left.
When you think about him, please remember what a cowardly worm he was. So many things you've told us about him show how worthless and selfish he is. But unfortunately you have swallowed some of his self-serving lies. You need to recognise how he has been gaslighting you.
You said he's decided he needs to do the right thing and stay with her plus keeps bringing up why he felt the need to leave as we were having a bad patch.
No! Far from "doing the right thing", he's walked out on you and his two existing children. Yet he has the cheek to try to make you feel guilty.
He has inflicted horrific pain on you, yet he dares to get irritated when you can't hide your natural grief.
He's getting pissed with me because at times we are getting on really well but then other times my emotions kick in and I pull away and stop contact until things are smoothed over again. I cant seem to block out my pain to keep decent communication with him, but he thinks I'm just trying to be spiteful.
I don't believe even a man as stupid and insensitive as him can genuinely believe the woman whose heart he has coldly broken is "just trying to be spiteful".
Soon, I hope, you will look back on your time with him and feel deeply relieved that he is no longer in your life.
All of us here are sending you good wishes, and will be here if you want to talk things through again. Best of luck, OP. Good times ahead.