Andrew Burke attacked 28-year-old Cassie Hayes from behind, cutting her throat, in front of horrified staff and shoppers in Southport town centre on 13 January.
It was the day after he had been convicted of harassing his ex-partner, 29-year-old Laura Williams.
Burke, 31, from St Helens, pleaded guilty at Liverpool Crown Court on Monday to her murder and having an offensive weapon, namely a knife, in a public place.
“You perpetrated a cold-blooded execution in public for the purpose of revenge and in my judgment the killing was an act of unspeakable savagery,” said Judge David Aubrey, sentencing Burke.
“This was a cowardly act of revenge designed to kill and did kill a totally innocent person in broad daylight in front of, and witnessed by, so many.”
Prosecutor Gordon Cole QC said the killing was a pre-meditated and planned attack carried out in a “totally ruthless way”.
The court heard Ms Williams began dating Ms Hayes after the split with Burke.
Burke had repeatedly threatened to kill himself or Ms Hayes because he held her responsible for the end of the relationship, according to his mother Louise McConnell.
She alerted police to the threats he had made over the phone to her in January 2017.
Police also warned him to stay away from Ms Hayes in May last year after he was accused of harassing her.
Months later, in November, he was charged with harassing Ms Williams and convicted of the offence a day before the murder.
He walked into the Chapel Street outlet and attacked Ms Hayes as she assisted a family.
“It was immediately apparent that there was blood flowing and that Cassie Hayes had had her throat cut,” said the prosecutor.
He remained inside the branch and was heard saying Ms Hayes “had it coming for… what she had done to his family”.
Mr Cole said: “After this ruthless and chilling murder the defendant seemed to remain calm. He showed no emotion and remained seated whilst everybody else was reacting in one way or another to what had happened.”
Police arrived to find Burke lying on the floor. He told officers: “I was in court yesterday and I lost. It was either kill me or kill her, so I killed her.
“The only reason I wasn’t still cutting her when you turned up was cos you’d shoot me and I didn’t want to put you through that.”
Ms Hayes later died in hospital.
Her mother Tracey Hayes told Burke he should “rot in hell” for the murder.
“You took the most precious gift I ever had in my life, my beautiful daughter, my baby, in the most horrific way ever,” she said in a statement from the witness box in court.
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“You are the most evil, evil monster walking on this whole earth. You do not deserve to breathe fresh air.”
She added: “It has torn our lives apart… I hope you rot in hell you evil, evil coward of a man.”