When you choose the find another route at the bridge you will get the message about finding glowshrooms, visit the lizard in her ruins by the inner temple choice or the witch in her hut to ask about the cave, they will make the item needed to explore the underground by the cave when you have it equiped.
Is it in your equipment screen, the one you see when you click the wooden sword icon on the top screen? The equipments that you can equip are on there, if you end up selling or discarding them by accident, then i am afraid you will have to start a new game and report this problem to Hayo so that quest/key items/equipments does not get removed in any ways until you create the key/quest items to explore the place they needed to be used on.