Copying as a way of learning art techniques is a very old and acceptable method of learning!
The point is you are doing the right thing by being honest about it, you admit right out that this is based on copying. So there's nothing wrong. It's only wrong if you try to sell it off as original work.
And you have done a nice job of capturing expression!!
In college in some of my fine art drawing classes, we were told to get our hands on great master drawing books, or spend some time at a museum, and mimic what we saw, to look at their drawing technique and attempt to replicate it, and re-draw their drawing or a part of their drawing. Not to trace, but to draw it out for ourselves and make it be as close to their style as possible.
By doing that with a number of master artists on the human figure, it's a great learning excercise.
In a graphics class, the first week was drawing from comic books! The teacher said most people draw ugly people, and comics have created a style that is useful to anyone doing marker comps. And that interesting enough, was the beginnings of my getting into comics. My friends loaned me some X-Men and Teen Titans, and by the time I was done copying faces and clothes and such--I wanted to know what happened with the stories in there.
By doing this, too, you learn a lot about new things to bring into developing your own style.
And if you really want to improve your turtles, get some books on figure drawing. Practice drawing basic figures.
Sit with a sketchbook sometime and draw your friends with quick gesture drawings, or play a movie and every so often, freeze-frame it and do a one-minute gesture drawing. (That technique was suggested by Brian Stelfreeze, an awesome talent in comics!)
If you practice things like that, when you come back to drawing turtles, you'll start to find you get your own style coming into it. You can also get reference pictures from some other sources, like in this case, a photo of someone or a monk meditating. Then draw it so it is a turtle instead of a person.
Keep drawing! And keep posting pics.
