Don't worry about it, I uninstalled FNV4GB and reinstalled the exact same way i had it, which is the way Dirty Weasel Media showed in this video-
and now its been working fine every time I've launched it. I don't know what the problem was but now it seems to be gone.
When I try to launch FNV4GB through Mod Organizer it doesn't run and this pops up- the filename directory name or volume label syntax is incorrect, i'm not sure how to fix this, any help?