I feel like this topic is straying a bit. Anything! posted some very nice edits and advice for you, Dragen, which I would definitely study and consider. I have 2 other pieces of advice that don't adhere to this piece in particular, but have been very helpful to me as a pixel art beginner:
1 - Start by working on a smaller canvas. As multiple members have already stressed, pixel art is about controlling every pixel in the entire picture. This becomes exponentially harder the larger your image gets. It's pretty natural, when you're starting out, to attempt a pretty big piece; one of my first pieces was 640x480! But I used a lot of repeating elements, and it still turned out like crap, because I just didn't have the technique and fundamental practice I needed, and the piece was so large that it was impractical to do major edits. If you start with a smaller image, like 128x128 max, then you can repaint the entire image in a matter of minutes if necessary; this will allow you to test out big changes more quickly, and it will also force you to recognize how much a single pixel can matter.
2 - Start by working with just a few colors, like 4 max. The major advantage of starting this way is it will help you master the final values and contrast of your image long before you start working with the colors or palette, and by limiting yourself to 3 or 4 colors you can again make drastic changes to how your subject is lit without having to redo your AA or your dithering etc. It is very common for traditional watercolor or acrylic artists to do charcoal value studies before embarking on their final painting for exactly these same reasons.
3 - Look up references. Even if you are drawing a cartoon submarine-squid monster from outer space, something with next to no bearing on reality, it can be a huge help to look up references (squids, submarines, space shuttle, etc) just to make sure that you choose the right details to stylize or emphasize, and to refresh your brain about the basic forms that you're going to be trying to render. In your case, I think if you examine some photos of the night sky, the moon, and silhouettes of trees it should be pretty clear what corrections you need to make!
So, my advice is to put this piece on hiatus and do some experiments with smaller images, working from references (not tracing or copying photos, but making informed artistic decisions based on observation), so you can fine-tune your pixel manipulating skills, then return to it. You may find that you don't even need any critique, the errors of the piece will be clear to you, and you will be able to push it to the next level.
It is important that you think of this as taking a break from the piece, rather than quitting. Ambitious artists sometimes have to do this - there is something you really want to accomplish, but you're not quite ready for it yet, so you do some studies, and return to it when you're ready. This is totally natural and learning to embrace this process will make you a much better artist. Good luck!