You should off center your main focus point and maybe try to add some effects that compliment the main focus point to make it more appealing to the eye and eventually after you have fixed these two issues you will notice that there is not so much negative space. Also a smaller image size will give you less space to work with. I find it easier to start with smaller canvases and work my way up. I do so much in a small canvas and eventually the canvas size just isnt big enough anymore so i just make it bigger next time. Now I just start with an stock image and make edits to the entire picture and crop it at the end to what i feel looks best.