Hi Labug,
Thanks for your words of wisdom.

I have to get over this sense of impotent rage that I feel at the injustice of what's happened to us.

I am in a bad place.
The tax office phoned me a couple days ago, as did the child assistance agency, to say that I had not lodged numerous tax returns form 2004 onwards (WTF?) and would be prosecuted, plus I have to pay back almost $7000 in family benefits that H had been claiming under my name. At the same time I got called by S14's school to say that we were now $14,000 in arrears for his fees. D17's fees for this term arrived in the post the same day (another $7,000).
At her same time, H is sending me threatening emails saying that he won't attend Monday's mediation meeting (where we should address these issues) unless I confirm in writing that I won't request any documentation /analysis of his financial situation.

I feel like I can't take much more of this.

Seeing her driving our family car around our local supermarket was just too much.
The kids were shouting and crying about how wrong it was and I just decided to go and tell her.

No purpose served other than to vent.

Regarding your question;
"If this is OK for you, why then is it not OK for H to do the things he does?"

My view is that there are some things that are just so provocative that it would take a saint to ignore them. And some things are just plain wrong.

OW showing up to my daughter's music concert was one. OW driving our car (when we don't have a car to drive) to places she knows we frequent is another.

She can live her life, but just don't flaunt herself in front of me and the kids.

But yes, I'm worried about me too. Obviously I'm still enraged. It's time for it to be enough.