I noticed a lot of bad reviews because the film was confusing or pretentious. I felt the same way as soon as the credits rolled. But then I think I figured it out and found a deep sense of satisfaction and appreciation for the story.
Here goes: Part Two is real life. All these things happened to Aubrey Plaza's character (Allison). She was an actor with a manipulative director husband who cheated on her. She catches him in the act but is diverted by a black bear - seemingly ominous but a bit of a MacGuffin. (In part because there's only been one black bear fatality in NY in the last 100 years.) The next morning after a swim she has a clear head and comes up with an idea for a film. Part One is that film. She gets to star in it as the other woman - a version of Blair in her real life. She also got to increase the stakes with a pregnancy and heighten the drama with the unfortunate miscarriage, all the while painting Gabe - the stand-in for her husband - as a total piece of garbage, not to mention further vilifying Blair and absolving herself with the characters.
The order of parts makes it confusing and the actors playing the characters obscures the intent. I also can't account for the name of the films - the one they make in Part Two and the one she writes on paper in the final moment. But if it was too nicely wrapped up in a bow, that would make it less satisfying when you piece it all together. It's a little bit of a Fight Club moment. If you hate the film because it seemingly makes no sense, try out this theory and you may appreciate it more.
Also, Aubrey's performance was great. I thought that watching her meta-breakdown during the final filming scene through her husband and the film crew's eyes made it even more powerful. Aubrey was acting as Allison acting but informed by Allison's real-life. Just amazing.
Anyway, I hope my theory stands up. Feel free to correct, dispute, or add on.