Stuck in square resolution?

[charizardCory]

I don't know many technical terms, so hopefully this makes sense:

I changed the setting in game to "Windowed" and then made it smaller, just testing it for a little bit. This made the screen a square resolution rather than the rectangular, full screen resolution. But now when I change it back to windowed fullscreen, it's still in the square. There's just black space on the sides of my monitor where the field of vision normally stretched naturally before.

Even when I put it in regular fullscreen, the game only shows what's in that square area, only it stretches it out to fit my rectangular screen, which is even worse.

It seems like it might be a problem with the resolution size or aspect ratio, but I don't know how to fix it. Does anyone have any ideas? Do I have to change something in the game settings or do you think it's something in my computer's settings? I would really appreciate some help, because it's a little frustrating to have to play at a smaller resolution like this when I used to be able to play wide screen.