It might possibly be more commonly said for the second sentence. "Life is not a piece of cake; but this is my life - and a new one for me now." (or something like that).
Usually "a piece of cake" is a modifier of a specific kind of noun - like, my job is a piece of cake; or her life is a piece of cake; or their work is a piece of cake - denoting something is easy. A similar phrase is "easy as pie" ...
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