I just walked away from a friendship that was basically toxic for me to handle. I was the last of the friends that have stuck around for this girl. She at the time was seeking professional help. Basically what can I do for me to not feel like I failed to help. I know that she's not my responsibility and I can't get her out of the rut that she's in. It's making me feel bad. I don't know why but want to just let it go.

Sounds difficult. Maybe you should help her get professional help. This time MAKE SURE she gets help. It is not good to walk away from a friend but then again, I've never been in your position.

My advise is for you to get her professional help, even if you don't want to be friends with her, this won't make you feel guilty because she needs it and you have the clarity of doing all you can to help her.