Resolution not "affixing" itself upon booting up TF2, can someone help?

Recently I had downloaded updates for my computer, and upon rebooting it, the resolution had changed so there were black bars at the bottom of the screen. So I clicked my monitor button to see the resolution, and changed it back to 1920x1200 like it should be in the desktop properties.

Afterwards, I boot up TF2. Hmm. I notice it isn't making my mouse large like it should whenever you boot it up and it [switches] the resolution to low. After it loaded and went to the main menu, I noticed there were black bars at the bottom. Again, this should't happen, because TF2 should auto set the resolution of my computer to a low setting until I exit. I try to go into options and set it down to low resolution, and it makes the black bars even larger and the screen even punier. I tried setting it to 1920x1080 ( my computer is at 1920x1200) because it did not have 1920x1200, and still black bars. I tried windowed mode, but with the high resolution my computer runs like a first model toaster. I tried changing my computer's actual resolution instead of in-game, but it doesn't change the resolution in-game.

Bottom point is, I need to have this auto feature fixed so I can run my game without it running at 10fps. It used to run at 40-60fps.

TLDR: In-game screen resolution has not been affixing itself after Windows Update, causing major lag.