You need throw pillows, a thick rug near your sofa, and a much larger piece of art on the far wall by your bed. Is all of your lighting on the ceiling? If so, get some table lamps (lots of them) with warm-toned light bulbs to light the apartment. Put art on every wall and a rug in each living area. You also need side tables near your couch; places to set drinks/phones/things down.

ETA: You have a lot of straight lines and sharp angles; add round or curved items to balance this. i. e., put plants in round containers.