Prototyping real use cases is way harder than designing “Happy Path” screens

I’m building prototypes for a fintech product, and I realized something: designing the main flow is easy. Designing real behavior is not. What happens when a user skips a step?
what happens when data is missing?, what happens when they come back after 3 months? My prototypes look great when everything goes right. The second I try to map real-world use cases, my design files explode into duplicates and disconnected screens. I want a way to visually organize use cases, branches, and logic before everything becomes high-fidelity. Some place where wireframes, notes, and flows can live together, so im not guessing how the product should behave.

At the moment, my prototypes show UI but not product thinking.