You have to travel, a lot, inside of the cave, if you know who Saori is (the lady at home that sells you consumables), get at least 4 of them so you can properly move through the game, you'll need them from underground level 3 onwards...
Otherwise you can't continue...
Two tips I can give you is
- Memorize the entrance of some sites, a good trick is to memorize the colors of the walls and floor...
- Memorize which places took you to a place where you have already descended...
It's fine to load a prior save before you fallen somewhere, that way you can memorize where you want to go...