Nazanin Zaghari-Ratcliffe was detained in April 2016 while trying to leave the country with her toddler daughter Gabriella.
She had since been sentenced to five years in prison on unspecified charges.
Her daughter remains in Iran with family after authorities seized her passport.
In the most recent letter to her husband Richard Ratcliffe, 38-year-old Nazanin wrote: “A year has gone by since our embrace and kiss in London when I was on my way to Iran.
“Gisou (Gabriella) and I had packed only for a two-week trip to spend Norooz 1395 (Persian New Year, March, 2016) with our family in Tehran. A trip that didn’t return us back home.
“Our Gisou has grown taller. She now understands very well that her father and mother are not together. Father is in London and mother is far away living in a room where we have been visiting each other during all this time.
“The most painful part of this whole affair is that neither of us have witnessed our daughter grow up. Neither of us.
“The country we were once proud of has robbed us both of seeing the golden years of our daughter’s life. It has accused me of committing something I have not done. Shockingly it has condemned me to five years in prison, which I have to spend away from you and our dear Gisou.”
Nazanin said the first nine months of last year were spent in various solitary cells, many days during which she believed she would never see her husband again.
“My wails would go unheard in that tiny, dingy, cold, grey cell,” she added.
“No-one would see me scream for my two-year-old daughter who wasn’t in my arms. I was engulfed in surging waves of injustice and atrocity. I couldn’t save myself and no one was coming to save me.”
Nazanin added: “We have had rough days and have been apart, but our love has grown stronger.
“Today freedom has got one day closer. And I will return, once again, to the city that I loved. I will embrace you in my anguish.”
“But they have given us both a deeper understanding of many things, perhaps most what we mean to each other, which one day again we can share,” he added.
“As she also says: we shall overcome this pain. Freedom is one day closer. And there will be new journeys to explore. I echo that dream.”
'Struggling to walk'
Richard's mother Barbara, who still lives in Fleet, said she is concerned for Nazanin's health.
"Physically she's very poorly and struggling to walk," she told Get Hampshire on Monday (March 20). "She passed out again recently and her hair is falling out. She also needs an operation on her neck.
"We are very worried about her and the campaign to free her continues - it's the only thing we can do.
"It's taken over our lives and everything we do is to do with it.
"We're hoping she will be released soon but there is no indication that she will be.
"But we will get her out."
A change.org petition for Nazanin's release has more than 885,000 signatures.