Hey Zen,
I'm so sorry you are hurting.
Immediately after your break-up, you were SO strong and put together. Remember how I thought you were just posing as a codie on SR to do research or something?
Well, maybe you were not allowing yourself to truly mourn the loss of the relationship and it is all coming out now?
Also, being with the mutual friends, and discussing her over the weekend probably wasn't the best thing for you emotionally, especially if they don't know the full extent of what a mess she is. It can be upsetting because they are speaking about her as if she is a nice, normal, person, and then their reality starts to infiltrate your own. Before you know it, you are remembering her as a nice, normal, person, and consequently, missing her.(I'm not sure this is what happened with you but it has happened with me so I'm throwing it out there. )
As for the kid thing, you know she is using poor judgement introducing her children to the new guy. You had good intuition to stay away from the kids and did both them and yourself a favor. Your ex wants to hang out with her new guy, and get attention/laid. Instead of waiting for a weekend when the kids are with their father, she drags them along. She is putting her needs and wants above the needs of her children, just like she did when she got wasted and blacked out in front of them. Why? Because she is a selfish alcoholic.
I think revisiting SR will ground you in reality. A lot of this sounds like you are missing someone who never really existed in the first place.
Hugs