First of all, I love sea turtles, they're so cool ! What I see in your drawing is that you focused too much on detail without thinking of the shapes and structure first. Only after you figure out proportions and the gesture with simple shapes, should you move on to more detail. Then you can work out the structure or 3 dimensional nature of the shapes, then add the details. I did a quick sketch to show you what I mean, I hope it helps :)