I like the composition and the overall production, but I’m sorry to have to unease your mind by saying that in relation to standard songs in that genre and to the other instruments the drums are a little bit too quiet throughout (not by much for most of the song.) But when the left hand piano notes are played with more intensity toward the end it just completely drowns out the drums. This makes it stick out a lot as not being real because performers just wouldn’t do that and/or it’d be mixed to compensate.
Although, I still think you should have bought a mastering program instead of paying people to do it, at this stage anyway.
It is intended to have vocals someday, right? If not then it lacks a more concrete melody to be an instrumental as well as a variety of different instruments as Lenny pointed out.
My biggest issue though, is that the piano sounds quantized. If it’s purely your playing then it’s very tight indeed. If that is the case then it may be the velocities causing it to sound too programmed. And if the velocities are varied in a way that a human would strike the keys then perhaps it’s just the piano sample library that you’re using that lacks enough variation between the dynamic layers, in which case not much you can do with that 
The string writing is definitely something for you to give more thought to in your next song. I might suggest listening with headphones to both Vanessa Carlton - White Houses (Official Video) - YouTube and Vanessa Carlton - A Thousand Miles - YouTube and really take note and analyze aurally the panning of the strings and what each section of the strings is doing. One of the things I really like about her songs is the way the strings and instruments are arranged, they don’t just sustain all the time or play in unison with the piano/vocal melody. In the first song ‘White Houses’ listen to how the staccato/marcato sections are written in relation to both the piano and vocal melody and then the sustained parts. Neither of them play exactly the notes of the chords. You may not find it interesting and maybe most of her audience wouldn’t notice it directly, but I think it still adds the right kind of variation that seeps into our unconscious as a listener and as a composer songs like these really are very good to study, aurally and/or with sheet music.
The kind of production that goes into songs like that is insane though. For example some snare hits have more reverb on than others right after it in the next bar etc etc and small things like that, that do make the difference And I’m in awe myself at the standard of the composition and the production.
Of course the only thing that matters is that you’re proud of what you’ve done and learned. The effort you put in is very obvious so keep on writing and practicing and good going with this song 