Judges at the Court of Appeal in London overturned the “unduly lenient” six-year prison term imposed in the case of James Richardson and increased it to 12 years.
Richardson, 35, of Berridge Green, Edgware, north London, who stabbed popular barmaid Natalia Czekaj more than 20 times while they celebrated the New Year at their home, was originally sentenced at the Old Bailey in October.
He denied murdering the 34-year-old, but pleaded guilty to manslaughter by diminished responsibility on the grounds he suffers from alcohol dependency syndrome.
The Old Bailey heard that Ms Czekaj had been considering ending their turbulent relationship.
On the evening of the killing, they had both been drinking and jobless plumber Richardson took three knives from the kitchen and stabbed his girlfriend repeatedly while she tried to fend him off.
Richardson, who was four times the drink-drive limit, phoned 999 to report the attack.
His sentence was increased on Friday by Lord Justice Davis, Mr Justice Haddon-Cave and Mrs Justice Whipple.
The trial judge imposed an extended licence period of five years which remains in place.
Solicitor General Robert Buckland said after the ruling: “I referred the original sentence as I felt that it did not properly reflect the severity of the case.
“This was a sustained and violent knife attack on the deceased which involved her being stabbed in the back deeply enough to penetrate her heart and the offender sawing at her throat so severely that all the structures of her neck were severed.
“I hope this increased sentence gives some comfort to the victim’s family.”