@MFMods @hasunemiku2015 Thank you both for helping me, but I am apparently just dumb. The solution was to upgrade intellij. Since I've been writing Java code for a few years and I downloaded the IDE a long time ago.
This is how I figured it out:
I was getting very tired of the problem, so I figured I'd try Fabric and see what it was about, but when I tried to launch the game in Fabric I got a more comprehensive error telling me the gradle build wasn't compatible with my version of Intellij (which I find very strange, but such is life). With this in mind I updated and retried forge, and that was apparently all it took.
Thank you for your patience, wish me luck!