Whenever I flick my mouse fast, only in this game, it will sometimes jerk way more forward than I actually moved. Ive tried a few commands and things to fix this with no luck. Any ideas?

Are you using Source Unpack?
What are the specs of your computer?

Yes I am. I didn't previously have this issue. I've got an Intel i7 CPU, an AMD RX 480 GPU, and 8 GB RAM

I actually think I know what you mean. When you move your mouse but you turn way farther than you wanted to as if your sensitivity was much higher?

Are you running the 'normal' batch file for portal, or the one in the rinput folder called "Portal+RInput"?