Hm... Yeah, both those folders.
Since that didn't work, the next thing I would try is to copy your Worlds and Players folders to another spot, then completely delete "My Games/Terraria/Modloader" and "My Games/Terraria" (also backing up any World and Player files in the unmodded Terraria folder) also the folder "Computer > Program Files(x86) > Steam > SteamApps > Common > Terraria", then reinstall Terraria from Steam, then reinstall tModLoader atop that after running it to make sure vanilla Terraria works.
Then install just tModLoader, and load the game again. If that works, then copy your player and world files back to the right place, and load the game again. An "/!\" symbol will tell you what mods you had installed when you hover over it, so reinstall them until you have the same setup (or as close to it as you can, if a mod has updated for example) then try reloading your mods and see if the game works from that point, since by now I can only assume it's a problem with the Terraria installation itself or something corrupted in one of the folders.