Nursery worker Deanna Burrows, 19, suffered gaping scalp wounds when she was set upon by Lucy Riddington in Huntingdon, Cambridgeshire.
Ridding ton lured Deanna to an alleyway before leaving her drenched in her own blood after attacking her with a hammer.
The victim spoke out this week after attacker Riddington, also 19, avoided a jail term on a suspended sentence for the violent assault in 2017.
Deanna said she and Lucy were friends but that her attacker turned on her when she became jealous about Deanna’s friendship with her boyfriend.
She had been messaging Riddington’s boyfriend on Snapchat during their school summer break but said they were nothing more than friends.
One night, Deanna – then 17 – got a message asking her to meet him but when she arrived she was attacked by Riddington.
The attacker had used a tablet belonging to a relative of her boyfriend and posed as him on Snapchat to lure her victim in.
Deanna told the Sunday People seeing her attacker standing over her holding the hammer has left her “mentally scarred for life”.
She said: “My relationship with her boyfriend was nothing more than friendship.
“There was nothing romantic going on.”
When she asked Riddington why she was being assaulted, her attacker replied: “You know why.”
Deanna added: “As I scrambled to my feet I felt what I thought was water pouring down the side of my head but it was really blood.
“I set off stumbling for home. When I looked back, she was stood there staring at me with the hammer in her left hand.
“I’ll never forget that image. She looked like a horror movie serial killer.”
Deanna’s mum had heard her daughter’s piercing screams as Riddington attacked after she managed to call her during the assault.
The teenager, of Huntingdon, Cambridgeshire, spent a night in Addenbrooke’s Hospital where her head wounds were glued.
She was later told she could have died as Riddington continued to attack her even as she was unconscious.
The victim said: “I’m not the same person since Lucy attacked me. I can’t go out alone in public and feel like I’m always looking over my shoulder.
“She could have killed me but she got off scot-free while I’ve been left mentally scarred for life.”
Police discovered four hammers in Riddington’s home in Huntingdon and found Deanna’s DNA on one.
In February she pleaded guilty to attempting to inflict grievous bodily harm with intent at Cambridge Crown Court.
Riddington was sentenced to 18 months in a young offenders’ institution, suspended for two years.
She was also ordered to complete 120 hours of unpaid work and given a three-year restraining order against Deanna.