Am I really leaving behind everything on the Depths 2 after I enter the boss room? I had left behind a blood donation machine and a full health pill and everything else!
Nope you can't revisit the rest of the level, not without some sort of teleportation. From the BoI Wikia article on Mom:
You can't leave the room without teleporting, as the door is gone, so make sure to check every room and pick up every item you want before encountering Mom! (A door to the devil room may appear after a teleportation.)