I know what it is

I figured out what it is.  About the domestic violence situation and my felt need to separate myself from her choice to go back.

Here it is:  For over three years, whatever situation she's been in, I've "been there" with her via the texts, emails, chats, and phone calls.  I've spent time wondering if she was safe, worrying about what would happen when he got angry at her again, concerned for the safety of the little children involved.

When she left, the relief and happiness I felt was because, in a way, I was leaving too.

And when she went back, I felt like she was asking me to go back with her.  And I can't.  I won't.  She may choose to go back, but I do not want to go back to hell with her.