Hello Evan,
Check if the app or game you're using has a setting like “Lock mouse to window” or “Confine cursor”, This is often the easiest fix. If that’s not available, try adjusting your display settings. Right-click your desktop, go to Display Settings, and make sure your monitors are aligned properly so the mouse doesn’t easily slip over.
You can also try switching the app to “Borderless Windowed” mode instead of full-screen, which sometimes handles mouse behavior better. If you don’t need the second monitor while using the app, pressing Windows + P and selecting “PC screen only” can temporarily disable it. And if none of that works, there are free tools like Cursor Lock or Dual Monitor Tools that let you lock your mouse to one screen.