Firstly, thank you for the reply!
Secondly, wow I guess I'm on the wrong version of minecraft! On boot up the bottom left corner indeed says it is 1.5.1. Wow I feel like a moron
However, this brings me to a seperate issue: MultiMC tells me the version is, instead, 1.5.2 and that even when making a new instance with my minecraft jar it says it is 1.5.1, which I'm guessing is caused by the jar/original folder. How do I update my original minecraft's folder/jar?
Should I just run minecraft on its own without MultiMC, update it, back up all my stuff, and then remake the instances with the updated game folder, I guess?
EDIT: Aaaaaaand yep! That absolutely did the trick! My original minecraft is now 1.5.2! Now I just gotta replace the intances's files with the new ones, update forge and mods, and test it out.
Thank you so much for the help!