Ok, I actually figured this out! I've seen multiple people on here have a hard time with it, and I'll explain my experience.
After many hopeless tries of trying to get this to work, I found a program called jarfix. Again, I was hopeless. So I tried it, I ran the exe and bam, the forwarder3ds jar opened magically!
I don't know what black magic is going on inside jarfix, it says that it fixed an environment path for me, but I've already fixed those and don't even see the change it made... It's black magic, don't question it.
https://johann.loefflmann.net/downloads/jarfix.exe