It looks like you are completely drawing every little outline then trying to shade according to your outlines. Pixel art, especially for little things, is more symbolic and suggestive than realistic and explicit. All the dark lines make your creature look small and flat. Try to suggest segmentation by shading with pixel clusters instead of outlining each section. It will help add depth. Also, all the dithering and broken internal lines add a lot of rough, messy texture.

It would also help to study an example image of the texture you are going for. Is it squishy like a caterpillar (https://i.imgur.com/6VanBOV.jpg) or armored like a centipede (https://i.imgur.com/PDIeYa9.jpg) or slimy like an earthworm (https://i.imgur.com/ZDX3GRv.jpg)?

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I'm still learning, too, so hope this helps you. Here's a quickie version of the ideas.(If you don't want me touching your art, I can remove the image.)