Okay so here are my edits:
Not the best but I hope it helps you make your sprite better even if 1% at least.
Editing your Sprite:
1. Refined the pose a bit and removed the details that werent helping me. It's always better to start with a good pose than getting into details right away.
2. Looking at the reference and selecting colours accordingly. You made everything too low contrast and quite gray. I picked one colour and then judged how dark or light other colours were in comparision -
https://alstoys.files.wordpress.com/2010/02/ssd8.jpgDo notice that even now I have just filled in the major masses and no details have been added yet.
3. Shadows.
4. Highlights
5. Add details if you want.
Then I made my own sprite following the same rules, just made the pose a bit interesting. I am not the expert and sure you can improve the sprite I made a ton, but again I hope it clarifies what I am trying to say and helps you.