This is a very simple and beautiful poem. I like the effect of the lack of grammar. This makes me feel like the poem is very unofficial and unplanned, the way Severus would have written it - even if it was not intended that way. I also find the way it has been written - almost messy and defying the commonalities of poems - to be almost enchanting, and a great testament to Severus and the way he was feeling.
However as much as I enjoy this format in the poem, and believe that it was done with the character's personality in mind, it is a touch difficult to read. This could be taken as both good and bad critique, I suppose, as it shows how much you have thought about the characterisation within the poem. But I also feel that it could have been done better so that it flowed slightly, while still encompassing Severus' character.
I really enjoyed this poem. Well done!