I feel like this post can be more fitting in r/AmITheAsshole since you are asking for our opinions or justifications for the actions taken

And if you are wondering, yes, I don't think you're in the wrong. She was a selfish prick.

I understand the feeling of being used as a one-way diary or emotional support without receiving the same support back. Even if the person may be in their worst of times. It's draining. You had also already expressed your concerns about the three man friendship and all she needed to do was not to include her into the friendship you two shared (she can have her own separate friendship with the other person). She has not made any improvements to her behaviour and that's what lead you to stray from the relationship. This is completely justified. Lastly, her comments in the bar were completely ignorant of the situation. If she had not read the room, she could have apologised for it. But the way she responded seemed like she was completely knowing yet still wanted to make it about herself. "I paid... I just got...", all comments about herself and none to ease your discomfort. Your actions are all justified, I believe. In her text messages, it didn't seem like she was at all -or if in the slightest possibility that she was, truly- sorry for any of her actions. She's getting upset that she's no longer being treated as a princess and being consoled. She was not there for you when you were there for her. The friendship was one-sided. It's good you ended it.

Now, my advice. Find more friends who are better than this. The mutual friend and her boyfriend were such great people who understood the situation, empathised with you and supported you when you were uncomfortable. Keep them close.