Brilliant. Peter or Miles? or Gwen?or Mayday?! I kinda think Mayday is the best, but I'm a geek.
Everyone is encouraged to re-read pp.46-48 in 'Drawing on the Right Side of the Brain' and repeat this exercise as often as you want to to build both your perceptual skills and your mental L-to-R mode shifting skills. Everyone is also welcome to re-submit this assignment (decorating, filling with zentangle, expressing & interpreting the positive & negative spaces of the vase/face illusion) for a higher grade before the end of the semester.
Are you kidding me? You are SO hard on yourself. This is "tight." You are very meticulous and careful. Heck, I'm jealous. I'm not saying be conceited and arrogant, but you take such pride in your working and workmanship, you may as well allow yourself to be proud of the finished artwork too. Keep up your incredible dedication to the process and not only will you continue to produce great pieces, but they'll likely keep getting even better. Anyway, I'm really proud of your work, I want you to be too.