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Amateur opinion here. I like the above designs but they do read as armor variations for the same character. Might be a stronger difference if it was in color and you had different palettes. To me, concept art is visual problem solving with each entry being a mostly unique attempt at getting close to your creative objective. It's about choice. If I was on the other end and had to chose, I wouldn't feel like I was really getting a choice. Do you prefer the red mini-cooper with the cream roof or the green mini-cooper with the black roof? Your overall designs for these guys don't deviate much from each other. I do however like the contrast changes you made, that definitely helped quite a bit. You've clearly got the skills to do this. I believe that you can go all the way.
Sounds good - just keep at it. Don't worry about "letting us down" or any of that...it's just part of the process. Push yourself to think more freely and really be creative with your design work. Good luck!
100% agree with what everyone said. Keep trying and you'll succeed. In my opinion, your rendering skills far out match your drawing skills. Perhaps a more carefully structured drawing approach would help you as well.
It's good stuff! Don't give up!