Hours of rendering become a part of any artwork. The reviewing and looking back at what you have done is all part of the process. I found a good technique to use when you are almost completed, yet you want to make a few changes without ruining everything - is to use baking paper - or tracing paper and work out what you want first on the baking paper before you actually apply this to your original artwork. I used this technique with this drawing and I explored how the face would look if I changed the smile slightly. I'm glad I could take the creative risk first on the baking paper and confidently apply the idea to the original.
Below you can see further progress with the shell. The baking paper also works well to stop your hand from smudging your work.