Writing this as I listen:
The piano at the beginning was nice enough, but I think you could do more with it. Maybe add some chords or counter melodies/harmonies. I can understand if you just wanted that single melody, though. Once the rest of the instrumentation kicked in, the harp stuck out a lot. Seems sort of loud to me, but this is on a pair of monitors. It could also be that it has a fair amount of reverb while the rest is pretty dry. If you added reverb to it I'd tone it down a bit. If that's the sample's natural reverb, a little bit added to the drums and bass might help. That's not my forte though.
At 2:20, the acoustic guitar was exposed(moreso than it already is) and the strumming sounded very fake. As I usually record my own guitar when needed, I can't really help you with adding realism to the sample beyond telling you how I'd play it. When playing like that, a guitarist will usually be alternating strums up and down. As it is right now, it sounds like what would happen if you only down-strummed(something I often did when first starting out). I'd suggest adjusting the velocity and volume a bit so it sounds like down/up strumming, and occasionally accenting down strums that are on beat. Really, if you're going to go acoustic I'd suggest having someone record it or investing in a good sample. You could even pick up a relatively cheap acoustic guitar and learn yourself - Unlike a lot of instruments out there(brass and winds, violins/viola/cello, vocals) the guitar is relatively easy to pick up and play. And you've got a whole internet full of tabs(including ones for this song's rhythm guitar) to work with.
My biggest complaint would be the repetition, though. After the piano intro, everything just sort of looped with more elements being added every 4-8 measures. That's more of a dance/trance music technique that doesn't really work well if you're going for a ballad. It needs more variety, more dynamic variance. It needs a break from the usual chord progression. At 3:07 when the second electric guitar comes in, you had the perfect chance to kick everything up and go into a heartfelt original solo. I was honestly a bit disappointed that you didn't.