There has to come a point in your life where you look at your art and realize that it's your own. What I mean to say is that it's fine and all to look at other people's art, but the second that you start to doubt your own skills or question your efforts is the second that you need to stop.
Look down at your hands. Those are your hands. Close your eyes and just sit there for a few seconds. Those thoughts are your thoughts. You are unique and your art will be unique. It may be true that your work isn't as refined as Neil Gaiman's or Jamie Hewlett's. Don't worry, that comes with practice and experience. Enjoy yourself. Art is for other people as much as it is for you.
When you feel uninspired or doubtful of yourself, try not to compare your work to other people's, especially those with more experience than you. Instead look back at your other works and see the improvement. More often than not, there will be improvement.
Strive to improve. Strive to excel. Strive to be as good as your favorite artists. Don't strive to BE your favorite artists. It's something that I'm still trying to get into my thick head.