Sometimes an image will indicate where the light source is but when it doesn't you need to choose where the light/highlights will be. Coloring the image with consistently keeping in mind where your light source is should be the ticket to that dilemna.
As far as your mojo needing a boost, just remember that crafting isn't a matter of life and death, and in that there are no "wrong" answers. I think that a person can get too concerned with producing something that is "perfect" that they lose their joy for creating anything. It can be a challenge to silence your inner critic, sure, but you can also tell it to be quiet and stay quiet so you can keep doing the things you love.