on Fiction University:
The act two disaster shows readers what your protagonist is truly made of.
Since I enjoy putting my characters in terrible situations, the act two disaster is always a lot of fun to write. Writers who want to protect their characters probably won’t find it so entertaining, but remember—this is the moment that allows your poor protagonist to become the person they want to be. So it’s good for them.
All through the middle the protagonist has been trying and failing, feeling the pressure, ignoring their flaw and the lessons the plot has been trying to teach them all book. Just when things are the most dire, something happens that causes catastrophic failure—often something they did, or didn’t do but should have. (Adjust this to fit the scale or scope of your story. What’s catastrophic in a science fiction epic is different from what’s catastrophic in a romance).
In cliché speak, it’s the darkness before the dawn. It all becomes too much and the protagonist feels like giving up, but finds the strength to carry on. They realize the only way to succeed is to face the problem head on and do what they’ve been scared to do all along.