First of all, as someone who has an unbridled obsession with the order Testudines, I must say that I approve of your subject matter.
You've got a good start going, but there are some pretty big issues with proportion and form. I couldn't resist doing an edit, so here's the ridiculously messy result of about an hour and half of scribbling over your work:
There's a lot wrong with my edit, so I wouldn't read into it too much, but hopefully you get the idea. Here are some of the main things I see:
-It looks like you went into detailing before you got the main forms down. This flattens the whole thing, especially under his "chin," like you noticed. Focus on giving him a depth before adding all the little bumps and wrinkles.
-Another issue in the area under the chin -- you added a scale-like texture when the reference actually shows more of a wrinkled-looking skin. If you google "tortoise skin" and scroll for a while, you'll find that many tortoises/turtles have a lot of areas bare of scales (which helps them be flexible and articulate enough to do things like tuck their heads into their shells).
-A lot of the colors are very close to each other in value, and more contrast would help make it pop. Particularly brighten the highlights on the top of the head because that would add more depth and also makes the lighting clearer.
-The proportions are very different from your reference's in some spots. The ones that stick out immediately: his nostrils should be bigger (they're about the same size as his pupils in the reference), and the whole mouth/nose region should take up more space; the eyes should be further apart and less centered; and his head should curve in a tiny bit towards the middle before going out again.
-I don't think mirroring and then going in and changing some parts is going to work in the long run for this guy; there's a bunch of asymmetry in your reference, and adding in the extra work to show it all would make him look a lot more life-like.
I know I'm forgetting stuff as I write this, but I'm tired and probably getting less and less coherent. Hopefully something in that garbledook was useful to you. Keep going with this!
