David Anderson strangled 36-year-old Louise O'Brien at their flat in Perth in April.
Miss O'Brien's body was later discovered on a bed with a heart-shaped chocolate beside her.
A court was told her family was still too upset to give a victim impact statement.
Anderson, 41, had originally faced a murder charge, but prosecutors accepted his guilty plea to a reduced charge of culpable homicide.
The High Court in Glasgow heard the couple had been together for about a year before the killing.
They were known to fall out, but Anderson was described by one person as the "best boyfriend" Miss O'Brien had met.
In a text message sent weeks before the attack, Anderson told her: "I've given my life to you. I have no interest in anyone else."
However, the call centre worker later told a colleague he was going to leave Miss O'Brien and move to Dundee.
Prosecutor Bernard Ablett said the exact date of the killing was not known.
Ms O'Brien was last seen on 24 April, before her body was discovered four days later.
During that time, Anderson called in sick to work claiming he had an upset stomach.
He was described as "sounding lost" and not his usual "chirpy self".
Anderson made a series of web searches including: "If you are accused of murder, are you automatically put in prison?"
The court was told that he contacted his mother and told her: "I've done something terrible and I've been unable to deal with it mentally for days.
"I tried to hurt myself because I could not deal with it.
"I don't want to tell you, but it'll come out anyway. I have broken the law. It is very serious."
Police were alerted and officers discovered Miss O'Brien's body in a bedroom.
Mr Ablett told the court: "A chocolate sweet wrapped in red foil and in the shape of a heart was found on the bed beside the body."
Miss O'Brien's mobile phone was later checked and a number of messages to her son were found.
The advocate depute said: "It is suspected that Anderson was responsible for sending these texts to give the appearance that Louise was still alive."
The court was told the exact cause of death was "less clear", but that Miss O'Brien had injuries consistent with "manual strangulation".
A man killed his partner before confessing to his mother days later he had "done something terrible", a court heard.