Just grab the doll and run through the exit door if you want the hidden memory. You can't miss it because you need the fuse box part to leave the area. Olivia is the doll which is nailed to the wall with the fuse box part (in the dead end where you pick up the flashlight, behind the doll's golem). Basically you only have to do one thing: look at the dolls Golem while you hold the teddy bear in front of you like a shield.)
Grab the teddy bear again and enter the dead end where the flashlight is. Leave it there until you can open the big steel door. He will now lie on the floor in front of the cupboard in the cave. Before you go through the cupboard into the cave, grab the teddy bear (not the one with the long ears!) and go with him (while you hold him) through the cupboard door. If you still don't know what to do, just do the following: The doll's golem itself gives you the tip for this achievement (teddy bear), as well as the memory "My poor Bear". Cross the hanging bridge, use the metal wheel with the door and confront your fear! You'll find an Inhaler and the Memory "My poor Bear". Grab the metal wheel and throw the doll's head into the 4th mirror there. Once you've aligned all the mirrors, you can go to the back of the cave. Align the first mirror until you can find the Memory "The Lighthouse" on the back of the second mirror. Grab the doll's head and carry it to the other side of the bridge (don't fall down or the doll's head is gone). Cross the hanging bridge again and collect the mirror part which is stuck in the wall. collect the Inhaler in front of the moveable mirror and then activate it to get another note "mirror riddle"
Walk to the big metal door, grab the doll's head and throw it in the mirror Touch the big mirror and you'll get the note: "the mirror in the cave" Since you can't carry both, just throw one doll through the exit and take the other one with you. Olivia is also necessary to get the "School Vacation" memory. The Teddy Bear is necessary to defeat the Doll Golem as well as get the "School Vacation" Memory. You can do "confront your fear" and "i won't leave you" in one game: The teddy bear and Olivia are necessary to get a hidden memory!