My first impressions (I don't have Sibelius so I can't access the score):
Please, if you can, change the sound patch for your instruments. They sound, uh, something terrible, let alone anything near a violin, viola and cello.
The melody is very promising. I wish you'd develop it more coherently.
You could pull off the shift from your strictly tonal theme to polychords and dissonance better if you incorporated a bit of the latter into the chords of the first theme, and in the development played around with that theme in the same tonal manner you presented it in.
The violin, viola, and cello all have much, much larger capabilities and ranges. I hardly even hear you going to the E string on the violin. I'm a violinist, and I could probably play most of what you've written for it at least two or three octaves above. It would make it so much more interesting if you worked with that a lot more.
Keep it up - with work (and hopefully private instruction by a composition teacher), you will develop into a very good composer! :P