Hello from France! Yes, the book is "A child's garden of verses", and "my shadow" is about a little boy who tells all the strange things his shadow does.

I had this book, don't remember the poem specifically (something with a candle?) But it's called a " Child's Garden of Verses"

Really good analysis. I also found it very interesting that the main character never spoke for himself, that we just heard from Dr. Jekyll oder Mr. Hydes actions by other people.