Here is my 100% Achievement guide and full walkthrough for Tell Me Why (Chapter 2).
This is story related and cannot be missed.
After the store, you end up at the cemetery.
You will walk up the only linear path, interacting with memories along the way.
(Note that when you get past the only gate, go to the opening on your left, then the next right, and behind the grave is a Collectible).
From this grave, head down the slight banking behind you, and find Mary anns grave which is infront of you in a gaggle of 3 tombstones.. hers is the left one.
Once interacted with, a long cutscene will begin and the achievement unlocks there after.
Go to 1:33:00 to see the whole cemetery section
Hope this helps!