Make it two object as the back and low part are two diferent part in your reference image. It will add a visible separation betwine the seeting and the back part that is curently missing.
Then try to get the top part at the right angle as it is going slightly forward in the reference and not in your model. This part is also thinier in the reference.
The seams need to be more visible in the back and you forgot the ones on the seat.
The border of the back and the entier seat part are ticker in the reference.
You forgot to model the metalic parts.
Edit: of course I looked at the link to the tutorial after I made my critic. It dont change a lot at what I said earlier, but now I know it was a beginer tutorial and from what I see you did pretty well. You even pushed the tutorial over its limit in some way already. The one point were you are falling a little short compare to the tutorial final image is the cushion seams part that arent deep enought in your version.
What I wrote earlier may be the way to go if you want to push the modeling part farther.
In case you dont know how to cut your object in two. Select all the face of the back part in face mode, hit [p] and choose by selection.