I think you're working to large for your current skill level. Try wokring at around a 30 pixel head (28-36 pixels tall, roughly half of this size). That will give you enough pixels to try different things but not so much that you get lost in the fine details.
Straight ramps for skin colors don't work that well. You can get away with it if the face is very small, or the image is dominated by strong contrasts with hair, clothing, or background, but a zig-zag in the hues/saturation seems to work better. Caveat1: this also depends on the intent of the piece.
An edit. I think there's still something off about the perspective, but I'd have to dig out some references to check it against. And the jawline needs to be fixed, but that depends a lot on the hair and/or neck.
Hope this helps,
Tourist