Yes you need help!
Arne's version is pretty advanced in a beautifully raw way.
Its probably hard to grasp the nuances of what he's done here so I suggest you save this for future reference and keep inspecting it as time goes on if you stick with pixel art.
I don't have time to edit so I'd say forget about leaf-level detail and anti-aliasing for now.
Try following these points and post another attempt:1) You're working too large IMO. Arne's edit was posted 2x scale. You then took that and started editing directly instead of reducing the size by 50%.
2) Your palette has no mid-tone green. The jump from the "mid" (slightly less dark), to the next brightest colour is too great so its going to make it pretty challenging to work with. Try going cooler (blue and usually less saturation as in Arne's) in the shadow areas too. Arne was a bit mean to throw in a cool highlight (hue bounce is more advanced) so you might find it easier to blend to a yellow sunlight.
3) You're tree-top structure is out of perspective, the bottom is not round enough giving a cardboard cut-out effect.
Maybe try drawing yourself some elipses (on a layer above if you have that facility) that fit into the iso-square and use them as a guide at different heights of the tree.
Someone (Cyangmou?) did a tut of that method but I can't find
4) Be consistent with you're light source, it can't be shining on the left and the right at the same time as the top.
5) As tempting as it is to highlight all over the shape, generally reserve your brightest highlight for the top clump where the light is hitting directly. Likewise, reserve use of your lighter greens on the outer edges to smaller clusters for just the odd leaves that break away from the clump to catch light.
6) Don't use single pixel noise. I personally like to start with a larger square brush to roughly block in the forms, then maybe draw the shadowy "outlines" of the main clumps using single pixel lines (or you can start with line-art if you like), then I use a 2px square brush to tap in the suggestion of leaves - tap sparingly and unevenly. At this point you should have a pretty good impression of a tree ready for pixel-level detail.
7) There's no way the trunk would receive that much light unless the sun is low (which colours don't suggest) or you've setup a weird spot light there. Look how Arne has more shadow and then bent the shadow around the shapes of the roots to accentuate the forms.
