Right off the bat, this gave me the feel of being submerged in an ocean -- the synth, even the piano riff and the heavy notes. When the drums kicked in, it felt like a submarine was being powered... (think the discovery of the airship in Final Fantasy X).
I thought for a moment there that when the drums kicked in, a slight bass track would've been much appreciated there -- that was what was missing. Once again, at 2:21 -- my understanding is that it could've come in and then resolved gradually.
Around 2:59 it feels like there's something looming above the submerged object/person.
Around 3:51.... that piano is beautiful! Because I'm finicky with pianos though, I'd have appreciated a little bit more reverb and a slightly deeper quality to the sound.
4:27: I can hear a slight bass, but something more definitive (like a subtle electric bass) would be useful here too. This was recommended to me after I made the same error with one of my pieces.
From 5:00 onwards, when that prettiness comes in, I can hear instrument buildup, which is great, but for some reason it felt almost anticlimactic. A little more rising tension towards the end perhaps, to convey the kind of joy from finding that beautiful thing maybe? :D
Now I realise that this is a programme piece, in that it tells a story, and it's in three parts. I'm fine with that. In fact, I've done programme pieces myself. : ) I really enjoyed it all the way through; though I still find that the last part relies on counterpoint a bit too much. I hear countermelodies, which is great for building up the tone, but one extra melodic theme and a little bit of chord variation would've made that last section spectacular.
I'm not the best with mixing help because I've a lot to learn myself, so much of what I can give is based on musicality. On that alone, I hope I've been of help!