I finished watching this last night and can actually understand why she kept going back to him.
First of all, she fell madly in love with someone very quickly off the back of a previous poor relationship and clearly had some underlying insecurities.
When he told her what he did, she did the right thing by telling police and says she had mentally prepared for that to be the last time she'd ever see him, but the police clearly didn't prepare her well for what would follow and that they'd be out until they had proper evidence. She couldn't simply just turn her phone off and ignore him so had to go along with it, all the while taking an increasing amount of drugs and alcohol.
I also felt there must have been an element of disassociation to the victim and the crime. She has been told it happened, but has never heard of or seen the victim before and has seen absolutely no evidence that a crime even took place - there's nothing for her to mentally connect with, if that makes sense?
Then when the body is discovered, she's removed from her job, the one thing that might have helped keep her grounded, and it's covid so she's even more isolated.
All that, fuelled by excessive amounts of alcohol and drugs, its' not that hard to see why she returned to him. Even if she was still scared.
I felt quite sad for her watching the scenes where she was trying to find the bike. It was so clear how unwell she was.
Although I did find all the filming of herself throughout a little odd, it did make it very interesting to see the change in her over the time.