Encrypted Persistence Not Working

I am new to linux, and have spent countless hours trying to get kali working. Live kali (version 2016.2) is currently functional, but i cant add persistence.:mad:

First i created the usb with Kali docs instructions (http://docs.kali.org/downloading/kal...ve-usb-install) on my Mac, which, as i said, was successful.
Using Kali's how to (http://docs.kali.org/downloading/kal...sb-persistence) i tried adding Luks encrypted persistence while on Kali Live. It seemed to work, but after rebooting, the file i saved to desktop had disappeared. I also tried using an ext4 filesystem rather than the recommended ext3 because other sites said they had more success with that, but it was still a bust. i tried each set of instructions a couple times, so i am fairly sure that it wasn't a spelling error.

Steps:

1. used fdisk command to add primary partition 3 (about 7gb) to my usb /dev/sdb
2. i then added luks:
cryptsetup --verbose --verify-passphrase luksFormat /dev/sdb3
cryptsetup luksOpen /dev/sdb3 my_usb (yes, i named my usb "my_usb" to simplify)
3. then i created and labeled the filesystem:
mkfs.ext4 -L persistence /dev/mapper/my_usb
e2label /dev/mapper/my_usb persistence
4. i then mounted, set up .conf file, and unmounted:
mkdir -p /mnt/my_usb
mount /dev/mapper/my_usb /mnt/my_usb
echo "/ union" > /mnt/my_usb/persistence.conf
umount /dev/mapper/my_usb
5. closed encrypted channel:
cryptsetup luksClose /dev/mapper/my_usb

then i created a desktop folder to test if it would save. When i rebooted into encrypted persistence in the menu, it prompted me to unlock the drive but it wasn't persistent - the folder didn't save.

i am sorry if i am making an obvious mistake, as i said i am new to this. any help would be appreciated as i am on the verge of smashing my computer.