So you've lost to Plantera and you're mad. Being mad is the key! Rage quit out of the game! Press Alt+F4 to quickly quit your game. However, this does not quit your game the normal way. All of your items and buffs on your character will save, but the world itself will not. That means that as soon as you lose to Plantera, you can Alt+F4, come back to where you broke your last Plantera bulb and it's back! This method is not guaranteed to work as Terraria might autosave if you are too slow, but this should work most of the time. Keep in mind that you won't be able to bring back your consumables this way.