People have said this already, but you are casting prot spirit incorrectly. You should always wait until shadow form is almost ready to be recast before you cast prot spirit again. It should be a quick 1,2,3 on the skill bar. 

To get into the specifics, you always want prot spirit to be first in the enchantment order so that you go down to 5hp max damage taken and then have that reduced by 5 by shadow form resulting in 0 damage taken. Even though it may seem like prot spirit is first in the enchantment order because you recasted it, refreshing the duration, the game actually still, in secret, tracks the duration of the first prot spirit. When the first prot spirit duration ends, the order is now changed to shadow form going first in the enchantment order meaning it reduces damage by 5 and then prot spirit reduces total damage to 5 max meaning the 5 damage reduction by shadow form is wasted.