See, this is where I gave it some leeway: I assumed that the back story for her was laid out in the two previous movies, so I went with it. If that's not the case, then I see where you're coming from.
Honestly, it's like being with a girl when she's going through her period. All the mood swings and crying in the movie are different phases of her period. It reminded me of the second girl I dated in high school. At the end of it you wonder why and if it's worth it for what you think is about to come, but it never comes.