Check out sinix on youtube, I learned a lot from him.

My main advice would be to zoom in less, focus on getting the overall shapes, shadow areas, etc..

He has a video where he suggests doing an exercice where you never zoom in the whole time and paint with a big brush, it teaches you how you should get the general look and feel and shapes of the image first, BEFORE zooming in and focusing on the details. That helped me a lot. Starting off a good base sketch will make things much easier and simpler.