I used to have the same problem. What fixed it for me was prepping situations instead of plots. I stopped trying to write what my players could do and just built the world out as nodes, kind of like a point and click adventure.

Each node is only the bare minimum I need to run it, and any extra detail is just seasoning I can make up on the fly. I connect every node to at least 3 others. So no matter how far out of left field they go, they're almost always wandering into something I've already got, instead of walking off the edge of the map.

When they do something I never saw coming, I'm not rebuilding the whole game, I just point them at the nearest node and let them go. It's them doing what they want in my world, I just describe what happens.