I see what the problem is here. First, fix the problems in form, right now the tree looks like it ate something. Get some good references and have a good idea of what angle you're looking at the tree at. That will make a big difference in where the roots should be and how much depth you'll need from the branches. The no single pixels is the best one to work from, since the form is the most obvious there. Then you need to get really used to zooming in. If you try to do the whole thing zoomed out you'll never get the details to look right, it will always look messy. Once you have the basic shape and shading the way you want then you can deal with the texture.