Very unusual poem considering we know she never married. Here she imagines being a wife as like being a “Czar”, an absolute ruler in full control of one’s domain, and she likes this feeling of power. She compares this to “the Girl’s life” which is an unmarried “pain” in which she has no control over her life and would be how she actually felt about her situation.
This poem speaks to the roles available to women through most of human history in that the best life might have had to offer is to be defined by their relationship with a man and, failing that, then being single is just “pain”. When Emily wrote this she was, and always would be, a single and unmarried woman which while she describes as being a “pain” she was also free to devote herself to her poetry, an art form which would allow her to (naively) imagine being a wife as being like a “Czar” of Russia, such as Catherine II. Of course being married is nothing like what she’s imagining here, but perhaps what she is getting at is that instead of being married to a man, she is married to her art?
As someone married to poetry, she is “Czar” of the domain of her imagination, she can conjure up anything she pleases and command her pen to do what she wishes – she is in total control. And before she was married to her art, perhaps she saw herself as a naive “girl” who had yet to take control of her life before she discovered the power of her poetry. Her poetry can take her all the way into “Heaven”, when previously she was merely stuck on “Earth” and the real world having to deal with real world “pain” and issues. And so she doesn’t want to “compare” her new life as a “Wife” to poetry because everything before that time “was pain”.