If you want to fill it out, don't worry about having something that makes sense, just some sort of shape in the bg, with maybe some light shading. I would suggest a curved line kind like this "(" that follows the same curve as the leftside of the mirror, drawn so that it's midsection is a little to the left of the rose. This is the tough part, where to position that "(" you want to make sure that is doesn't detract from the rose and is clearly in the bg but not too close to the mirror or else you'll still feel that negative space on the left.
Another option if you want to get the feeling of emptiness is to shade the left side very dark and move lighter as it approaches the mirror. Balance is key! lol I wish I could show you what I'm thinking

I hope this helps though.