1. You need to work on your text. (white is a good choice of color.)
2. No color scheme. Use colors that relate to the situation.
3. Your renders stand out, try smudging the edges.
4. When using "Gradient Maps" make sure you change the blend mode to "soft light" (thats what I use) and lower the opacity.
5. On most of them you have random brushes for the background. Try using brushes that blend in.
6. On the 2nd one it looks like you followed a tutorial. (i've seen it before) and I think it's great that you followed a tutorial.
Just keep looking at tutorials, and you'll get better.
~Inferno