Your interpretation makes sense, but before I read the cut, my immediate impression was negative. It's mainly word associations, I think: dark, winter, despite. (The other thing that's going on with that poem is that I first encountered it in Possession where its context is somewhat melancholy.)

If I were to write it in prose, I'd say something like: we're in love, and it's all wonderful, but the world outside is cruel and we have to be aware that this happy state is temporary, in the end there will be loss and death.