"This was the height of my eating disorder," Tammin began.
"I remember how sad I felt. Ha. Pictures lie, don’t they? I was dying to be perfect."
The Pretty Little Liars star continued to write about how a desire to be 'perfect' and being the victim of bullying led to an eight year long illness.
"The illusion of perfection that one cannot attain. For perfection is a road that never reaches a destination. I was not well for 8 years.
"8 years of torture I put myself through to remain slim after I had been tormented as an overweight kid."
Tammin continued to write about the importance of finding your 'truth': "I believe the more we talk about our truth the less isolated we become.
"We start to heal."
Back in April Tammin posted a smiling photograph of herself aged 13, and wrote an extended caption capturing that pain that hid behind that smile – the pain of a child who was relentlessly bullied by her peers, and the self-doubt that flowed from that relentless abuse.
The mother of two revealed the depth of her torment in a letter to her younger self, writing on Instagram of a scarring experience at a swimming pool.
"Hi, It’s me, your future self, I’m standing on the sideline looking back at your ripe, fleshy, hopeful face and I know what happens when the arm of the clock aggressively hits its top pinnacle," she wrote.
"The boy standing in front of you will break you. He will break your heart in ways you never thought possible. He will embarrass you as you feel the stares of awkward pre-pubescent males all stifling a hearty laugh.
"Not only does he not have any giddy interest in you, but he will verbalize in front of everyone that you are indeed 'FAT' and after he miniaturizes you, he will parade away.
"I see that girl. She will spend your school years antagonizing you, she will make it her duty to beat you down, trying to suck from a straw any self respect you might have.
"Not only will she steal your first real love, but she will celebrate it.
"But one day things will change. You will change."
This will be the first time Tammin has opened up about er eating disorder, choosing to share the empowering message on her podcast Women On Top.