In the pictures above, you can see what my problem is. Normally, when working with Godot, the collisions are very precise and clean cut. But oddly, in this project, they're sloppy, and the player (a Rigidbody2D) is never quite colliding correctly with other nodes.
Sometimes, it will get caught on imaginary collisions that don't exist, and sometimes, it will fall into crevices that aren't supposed to be there.
I was sure to turn on visible collision shapes to show that the node it's colliding with is simply supposed to be a circle collision. And yet, it doesn't collide that way.