Acualy I can draw a better tree in real life but I still make it crappy in GG .
Scan/take a photo of a tree sketch and show it in your next post, so we can at least see just how much help you need with fundamentals.
Pixel-specific, work smaller. Really. It's a lot less painful to work small, work fast, and if it's a lost cause, you can toss it without having spent a long time pixelling a large piece. The animated gif I posted earlier was 32x32, which allowed me to make a really quick animated example that wouldn't take too much time to clean up and refine. I'll say it again: work smaller.
Additional bonus to working smaller: you can actually look at proper pixel placement/relation, which is effectively the whole point of pixel art.
edit:Figured I could use the practise myself, so a quick example, this time with reference, and slightly bigger (still only 64x64, as opposed to the 200x200 pieces you insist on working on)
ref:
http://www.happyvalleyfdn.org/images/harris-walnut_tree.jpg
Again, this is sans refinement. It's just a quick block-in to show you what people are trying to say about focusing on the big shapes, getting the form down, instead of sweating the little things like lines, which are useless without structure. The main takeaways from this are:
1. work loosely for now, get the overall structure correct
2. USE REFERENCE. (and I mean photos, not illustrations that other artists have done)