If you have the Key of Regret, that makes things easy, as it unlocks the door to get the triangle you're missing .
Go do the mine minigame in the Observer's Shrine again. When the minigame starts, take the green door (upper right). After that, take the door to the right. Just to the left and up of where you appear next room is the next door you take (in the newest version, there are arrows pointing to the room, but they are missing in older versions XD)
There is where you'll be able to use that Key of Regret. Hope this helps :DDD