Sharon Hollinsworth repeatedly struck Andrew Oates on the head and body with a hammer at their home in Peterhead in October 2010.
The 44-year-old began digging a hole in their back garden in a bid to dispose of the body but could not go through with it. She handed herself into the police two days later, telling officers that Mr Oates had been repeatedly violent towards her.
Her daughter Natalie Hollinsworth, 22, and the couple's teenage son Christopher Hollinsworth, helped her plan the murder.
Christopher Hollinsworth texted his mother about the murder and discussed the use of a firearm and a hammer and who would carry out the killing. One text stated that the family didn't have "the time or money for a gun".
Evidence also revealed that Natalie Hollinsworth researched methods of killing on the internet, including the use of a hammer, and ways to dispose of a body.
And Sharon Hollinsworth was found to have researched how to buy rohypnol online and also texted her son about the murder plans.
The trio were all initially charged with murder and went on trial at the High Court in Aberdeen on Monday, but yesterday changed their pleas.
Sharon Hollinsworth admitted murder, conspiracy to murder and an attempt to defeat the ends of justice.
Natalie and Christopher Hollinsworth admitted conspiracy to murder. The Crown accepted their not guilty pleas to murder.
Advocate depute Kath Harper told the court: "Andrew Oates was a reclusive man who suffered from depression. He suffered from anxiety and anger management issues.
"At 3.30am on the 24th of October, the three accused arrived at Grampian Police headquarters and Sharon Hollinsworth told police officers that she had killed her partner.
"She said she had hit the deceased on the head with a hammer because he went for her son. Sharon Hollinsworth told the police that it was her intention to bury the body in the back garden. She confessed that she had started to dig but couldn't go through with it."
The court heard police were instructed to go to the family home at Fairwinds Place in Peterhead and discovered Mr Oates's body under a duvet with cable ties on his wrists. Splatters of blood were visible on the upstairs landing wall and the floor. A hole had been partly dug in the back garden.
Pathologists said Mr Oates had suffered at least 20 blows to the head and all the injuries were consistent with the use of a hammer, which was later found in the family home covered in blood.
On confessing to the murder, Sharon Hollinsworth described her life with her violent partner and said he was unpredictable and suffered mood swings.
Ms Harper added that the Crown accepted she had been attacked by Mr Oates in the past and stated that the mother-of-two had been assaulted by him in 1998 when police were called to the house.
However, a Crown spokesman last night revealed that this was only based on Hollinsworth's account as Mr Oates had never been reported to the fiscal for domestic abuse.
Scottish Women's Aid said many women see retaliation as their only solution.
A spokeswoman for the charity said: "Scottish Women's Aid would never condone violence of any sort but research tells us that women and children living with domestic abuse can find themselves desperate enough to see retaliating as the only way to protect themselves. This isn't
safe for anyone and we would urge anyone who finds themselves in this situation to seek help and support."
Speaking after yesterday's court hearing, Mr Oates's sister Sheena said the family had had to wait 14 months until they could bury their brother.
She said: "It has been very hard, especially for our parents, who are both in their 80s."
Mr Oates was only laid to rest in January this year following a lengthy and costly court battle over the release of his body because son Christopher was named as his next of kin.
Detective Chief Inspector Martin Dunn, who led the inquiry, said: "Today's outcome will not bring Andrew back but it does spare his family the ordeal of having to sit through a trial."
Lord Doherty remanded all three in custody and deferred sentence until later this month.