I want to explain to my 15 year old why her dad flew hundreds of miles away without even a phone call to tell her he was leaving. She's devastated
Velma...
Ugh. I'm really sorry you're left with dealing with the fallout. Regarding your 15 year old daughter, here's what I think.
I would simply tell her that her father is sick. And the reasons why he left without so much as a phone call have nothing to do with her. It's part of his condition, and I would encourage her to not personalize what he's done.
Of course, this is easier said than done. But it's also the truth.
I would also encourage her to attend Alateen, if she's ready to make that leap. It may be a comfort to know that many boys and girls her age have gone through something similar to what she's going through right now.
So, be there for her. Be strong for her. But be honest with her, too.