This isn't the main point of this post but the more I read MN, the more I realise there are so many people who are armchair psychologists and judgy fuckers of things they don't understand.
People are ableist as fuck with ADHD or other ND conditions on these boards. And now people are jumping onto OP because she mentioned she had BPD. As soon as I saw it, I knew the onslaught would come. And now it's turned the thread into what's wrong with people with BPD.
I bet you're the same people who posted be kind after Caroline Flack killed herself.
BPD people are not necessarily narcissistic at all, you cannot tar everyone with the same brush. It is true, they are much more vulnerable to abusive relationships. As are ND People. A lot of the time, they hurt themselves more than they do other people. They have had trauma, often as a child and BPD manifests as a result. Have some damn empathy. And no I don't have BPD. But I don't claim to fully understand a condition that I do not have. But I can empathise. It can't be easy to live with.
Now I'm not for one second suggesting the op handled it the best. I'm sorry OP I don't. I'm not saying you don't have a point about the little girl's manners and her mother's parenting. However, you had no right to change it over in someone else's house and then lose it as you say. Take responsibility for yourself. I have to with my condition and you do too. You can recognise the thoughts before the mist descends. I would recommend acceptance and commitment therapy. It's good for helping to recognise these things.
Once you've calmed down, have a really good think of your own actions. Hindsight is a wonderful thing but right now, it's good to reflect as you acted badly. I think you should apologise to your friend, even if you don't want to be friends again. You kicked off in her home in front of her children. You were very rude yourself!
I know it's hard getting a hold of your anger. I know with having ADHD, it's not easy. But you have to take responsibility for yourself and learn from it. Sometimes, it is you I'm afraid.
Good luck to you op.