The 38-year-old actress - who married the US chat show host in 2008 - credits her partner with helping her to fully overcome the anorexia that gripped her for much of her 20s.
She said: "I thank God for Ellen every day - she has enabled me to be exactly who I am. We first met in 2001 when I weighed 168lb, but she says she never saw me as heavy - she only saw the person inside.
"It's ironic, really, that I tried so hard to present myself as something I wasn't when all I ever wanted was to be loved for my true self."
Despite being openly gay since the age of 16, Portia admitted her mother encouraged her to be "secretive" about her sexuality in a bid to land acting roles.
She explained: "I had come out to my mother when I was 16 - but she had always encouraged me to be secretive, especially with anyone who could advance my career."
However, the former 'Ally Mcbeal' star's mother is now "proud" of Portia and the "profound love" she has for Ellen.
In an extract from her book 'Unbearable Lightness: A Story of Loss and Gain' published exclusively in You magazine, Portia added: "My mother was there on our wedding day and she couldn't wait to see me in my Zac Posen wedding dress - a vision that she could never have imagined after learning that I was gay.
"She told me that she was proud of me. And she couldn't stop smiling at her healthy daughter who had found a profound love with another woman."