Most journaling apps are built with features you do not need when you are overthinking. They add friction that works against you:
Logins and passwords. You are in distress and your brain is racing. You do not want to remember a password or wait for two-factor authentication. You want to write now.
Notifications and syncing. Apps ping you with reminders, sync across devices, or notify you about new features. Each ping takes your attention away from the actual work of externalizing your thoughts.
Data collection. Many apps collect metadata about when you write, what you write about, and how often. They sell that data to advertisers or use it for engagement metrics. Your overthinking thoughts are not a product to be monetized—they are private.
Internet dependency. If the app requires internet or connection to a cloud service, you are adding another point of failure. When you need to journal, you need it to work now, not "once the connection resets."
Design choices that distract. Some apps are visually elaborate, have themes, or encourage you to format your writing. When you are overthinking, you do not need a beautiful interface. You need simplicity and speed.