He continued by saying that when he was 20 he met a woman and fell in love.
"The passion between us would take us both to some of the most joyous highs, but also to moments of jealous bouts of insecurity, and volatility," he wrote, now aged 28.
He continued to write that he "takes full responsibility and accountability for the hurt that I inflicted during my relationship with my past partner – emotionally, mentally, and yes, physically".
Jenner admitted to an argument that escalated, saying he threw a phone that hit his ex in the face.
"In a moment of frustration, while I was standing in the hallway and she was in our bedroom, I threw my phone aimlessly and it hit my former partner in her face. I froze in a state of shock and horror as my then-partner screamed in anguish, her eye immediately swollen shut from the impact of the phone," he wrote.
"It's a moment that I will regret for the rest of my life ... I am responsible for the pain that I caused her in that moment and beyond. And it is something that I am still working on forgiving myself for."
While he doesn't mention names, he does say that the relationship he is talking about "was spoken about late last year".
In November, Benoist posted a 14-minute Instagram video where she spoke about being a survivor of domestic abuse. She even provided the details of the abuse, from emotional manipulation to physical violence.
"I learned what it felt like to be pinned down and slapped repeatedly, punched so hard I felt the wind go out of me, dragged by my hair across pavement, headbutted, pinched until my skin broke, slammed against the wall so hard the drywall broke, choked," the Supergirl star recounted.
She also went on to explain what happened after an iPhone was thrown at her, breaking her nose and injuring her eye.
"Deep down I never believed he would change, I just fooled myself into thinking I could help him ... Someone had to let him know his behaviour wasn't OK, and who better than the one he was taking it out on?"
In Jenner's post, he claimed the abuse came from both ends, and they started seeing a therapist together, but weren't able to get out of the "toxic cycle".
He also claims his former partner mentally and emotionally abused him, stating he "was made to pass" on acting jobs over jealousy of female co-stars.
Jenner claims he was "made to feel guilty for working as I received phone calls from my partner while I was away, threatening self-harm out of depression and a deep-seated fear of abandonment."
He says he was scratched, slapped, and punched in the face and he suffered a broken nose.
"I was physically assaulted in the shower, leaving me with a traumatic injury that I do not want to delve into at this time."
Jenner and Benoist worked together on Glee. They were married in 2015 and divorced in 2017.
Benoist welcomed her first child two weeks ago, a son, Huxley Robert Wood with husband Chris Woods.
DOMESTIC VIOLENCE - DO YOU NEED HELP?
If you're in danger now:
• Phone the police on 111 or ask neighbours of friends to ring for you.
• Run outside and head for where there are other people.
• Scream for help so that your neighbours can hear you.
• Take the children with you.
• Don't stop to get anything else.
• If you are being abused, remember it's not your fault. Violence is never okay.
Where to go for help or more information:
• Shine, free national helpline 9am- 11pm every day - 0508 744 633 www.2shine.org.nz
• Women's Refuge: Free national crisis line operates 24/7 - 0800 refuge or 0800 733 843 www.womensrefuge.org.nz
• Shakti: Providing specialist cultural services for African, Asian and middle eastern women and their children. Crisis line 24/7 0800 742 584
• It's Not Ok: Information line 0800 456 450 www.areyouok.org.nz
