Nicol's point about spending more time with the instrument and less with the music is very good advice. There's a balance that needs to be struck between absorbing music, creating music, and just playing.
"Knowing your instrument" means being able to hear, internally, what you are about to play. This is partly an aural skill, but it's also visual. Chords and intervals all have specific shapes or 'contours' to them that you become more and more familiar with. For example a fifth interval has a distinct, strong sound, but it can also be identified by the way it is played with the thumb and the fifth, giving it a symmetrical look and feel.
"The Skycity of Bhujerba" is probably the most complex thing I've written for piano, and there was a lot of work and frustration in getting to that point. When you start an arrangement of a piece, it's easy to give up, because the original music has the most dominance over your creativity. Your own musical style and ideas can't seem to get through, and like Nicol said, you restrict yourself. You have preconceived notions of what makes the song 'the song' - for example in "Bhujerba" the first thing I tried working around was the bouncy chords of the original arrangement:
YouTube - Final Fantasy XII Music - The Skycity of Bhujerba
Then when I cut out that aspect of the song and focused on the sweeping melody, it suddenly made sense to slow the song right down and let it flow into both hands. The "trick" to it for me, was to not listen to the original song for a while. I just memorized the melody, (by ear rather than sheet music, so it wasn't exact) and then as the weeks went by, I managed to "absorb" that melody into my own musical "idea bank", and suddenly I was able to play around with it a lot more freely.
Another roadblock was the pressure to make it sound like "a Piano Collections song". But again, this dissipated once I took a break from listening to Hamauzu and Hamaguchi.
With music, the brain remembers what it likes, so the original Bhujerba melody became my own melody, infused with all the other music floating around in my brain, with small differences from the original.
My compositions start with me recording ideas into Cubase. However, a big trap you can fall into is writing on the keyboard too much - it feels like you're making a lot of ideas, but it's important to start writing on paper. Don't worry about writing the work if it's unfinished - the point is it gives you a closer look at your arrangement, and the mental exercise squeezes out great little ideas you may never have thought of.
Sometimes you write a great section that doesn't fit into the rest of the arrangement, no matter how hard you try. Don't get too hung up on having to cut things out - otherwise you'll drive yourself mad re-writing the whole song just to include one new section. There'll be a place for it somewhere, maybe in a different piece.
DISCLAIMER: I don't follow this advice as much as I should. The "Bhujerba" you've listened to is the 5th iteration
Oh and Hikaru is right about the singing. It's the quickest way to 'solidify' an idea in your head especially if there's no instrument nearby! I often play chords and experiment with melodies with my voice.
Anyway I hope that helps you out. Keep listening, absorbing, playing and writing. Creativity develops at an uneven pace, but it never stops. Remember that even when you simply hear something new that inspires you, it's developing. And creative moments usually follow, and they are sweet indeed!