I agree with nhomas on the need of a driving beat, but I'm concerned with your strings.
You had a lot of double stops and the occasional triple stop in your violin parts, which, to my limited knowledge, are hard to pull off smoothly and make it sound more "accented." I'd split those up into Vln. I/II, or maybe have divisi. Divisi will make it smoother, I think, but will make it softer. As for the question about the range of a violin, I'm not too sure about it, but I think it is possible for them to go that high. Those arpeggios in the second to the last measure with the octave double stops will be hard to play I think, so yeah, I suggest you split them up with Vln. II or use a divisi. When Vln. I starts those arpeggios, it might make a nice effect to have the other strings tremolo their notes, or maybe play eighth or sixteenth notes, idk. It's fine as is, though.
Or, you could write it for a synth with a string voice and lessen complications, but I think the string ensemble would sound better.
You might want to look into putting part of your piano parts into the suggested guitar sections, because some of the parts are hard to do because, depending on the length of the pianist's fingers, they can extend to around ten steps (like C to E), and it gets harder because of the chords being played.
I don't know many singers who can go that high at the end (maybe Mariah Carey or Whitney Houston, haha), but a rule of thumb is that an A above the staff is usually the highest? You may want to ask about that, I'm not entirely sure.
Overall, I liked it. The chord progression and the melody worked nicely. I liked the little string feature in the middle.