The 60-year-old is now facing a life sentence after the court heard how he killed Angela Conoby as they faced losing their home amid mounting debt.
Stalgis took the case to a trial despite where, by his own admission in the witness box, all the evidence pointed against him.
The cleaner claimed that he could not remember carrying out the fatal attack or why he would want to hurt Ms Conoby.
Leeds Crown Court heard Ms Conoby, 55, may have been killed around two weeks before her badly decomposing body was found in the property on Berekley Mount, Harehills, on May 20.
A police officer forced open the door to the property after the victim's sister, Michaela, contacted police worried for her safety.
Footage from the officer's body camera was shown in the courtroom and Stalgis can be heard repeatedly saying "I lost the house", "she didn't know" and "I'm sorry".
At the police station Stalgis confided that he had "done a really bad thing" and asked for forgiveness "for killing the woman he loved".
Officers also discovered he had cuts to his left wrist that may have indicated a suicide attempt.
Later, when he was formally interviewed, he made no comment to the police's questions.
At the house, police found a pile of letters from solicitors and an apparent suicide note written by Stalgis which was addressed to Ms Conoby's sister and her partner.
The letter, read out in court, stated: "I lost the house and I killed the love of my life. I cannot live with the shame, please forgive me if you can. I deserve to rot in hell. As my friends and family I'm so, so sorry to put you all through this. I love you."
During the trial Michaela Conoby described how she became concerned when her sister failed to answer text messages and phone calls.
She rang Stalgis and he told her that Ms Conoby's phone was broken and that he was going to buy her a new one.
On one occasion Stalgis claimed his partner was out shopping at Morrisons when her sister asked to speak to her.
During cross-examination, prosecutor Jonathan Sharp asked Stalgis: “I am failing to understand how it was that you came to kill, and when it was that you came to kill Angela, having been through all the evidence. Can you assist us any further?”
Stalgis replied: “I am sorry I can’t remember when I did it. I can’t remember whatsoever.
“I know I must have done it because all the evidence points in my direction. But I can’t remember doing it.”
Mr Sharp continued: “You intended at that moment to cause serious bodily harm or intended to kill her.”Stalgis replied: “Not knowingly. I would never hurt Angela in any way, not knowingly.”