Girlfriend went back to toxic ex. Why?

Well long story short. I met this girl, we connected very well. Hung out, met each others family, just enjoyed each others time. Sadly, there was a bunch of red flags I stupidly overlooked. She has been seeing this guy on and off for about 6 years, out of that 6 they only dated around a year or two. They both would go out and date other people. The relationship is very toxic. Well anyways the ex is bum who doesn't do anything. I graduated college and already in my career. The girl is decently young, around 20. She went from very hot, meeting her family, just being happy, then out of no where she became cold. She said she wanted to be friends but we still could "mess" around. I knew her ex probably came back in the picture. MY questions are: 1) why did she go back 2) why would she want keep messing around and 3) what should I do? I stopped contacting her and just her live life.
Girlfriend went back to toxic ex. Why?
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