I don't understand this sentence (neither the English nor the Swedish one). Why is there an a / ett ? I consulted the dictionary and darkness is uncountable. I can't imagine how one can count darkness. Why not just My sorrow is like the darkness or My sorrow is like darkness?
Uncountables can be counted with poetic license, if you're using shorthand to e.g. talk about a type. For instance, there may be two types of darkness, and in a sentence like this, "a" darkness then makes sense.
That said, I think this sentence is a horrible fit for a beginners course.