Don't beat up on yourself for trying to help. You've learned what you need to from this incident. I used to do the same thing in my family. Always tried to be the mediator or protector in relationships gone bad. In the end, it was too stressful and harmful to me so I had to move far away from it. I still get phone calls about the drama on a regular basis, but at least it doesn't interfere with my life every day like it did before.

There are a lot of sensitive people who have many problems because they feel so much and care so much about other people's feelings...to the detriment of their own well being. Hence the whole boundaries cliche.

You're not alone in this phenomenon.