Hi Matthew,
Firstly, it's important that you ignore Justin's shameful attempt to undermine you. He's been writing music for a very long time, and he should be giving you advice not insulting you! I hope he hasn't put you off writing music, you must keep going as it's clear to me you have potential.
Very nice relaxing piece with lovely harmonies. It is a tad simplistic yes, but it works very well imo. My favourite part was the section starting, around 2:59, as well as the following section with the alberti bass. I think this section could do with a few supporting chords in the right hand however, although you do this later beautifully with the choir.
From the section starting at 5:21. I think you could work a little bit more on the transition. It's a very sudden change and is a little bit too suprising if you get what I mean! You could try a rit. slowing down to the end of the section, and then having the following section at a slightly slower tempo.
One other thing! At 5:31-5:33, there is a descending bassline that abruptly ends. I think it would work better if you added low C in the bass to support the transition to your C major chord.
Although I like your style, I do think you would benefit from adding some more complicated, 'thicker' harmonies. This does not necassarily mean changing every chord you write, but simply altering a few of them (eg sus chords, 7ths) although I'm sure you've put a few less conventional chords in at some point.
Also, try putting some tempo variation into your sequencer program! it will give it a less mechanic feel performance wise. Eg create subtle rubato by slowing down the tempo towards the end of a musical phrase and returning to original tempo at beginning (or slightly after the beginning) of the new phrase.
Keep writing, and post your results for some more feedback.
