Te Hiko told the jury he had been on the verge of entering rehab at the time of the attack and planned to marry Thompson when he was clean.
However, he became enraged with her and demanded to know who else she was seeing when she told him: "How do you know I haven't replaced you already?"
"I smacked her clean off her seat, smacked her into the next room screaming, 'f*** who is it?'" Te Hiko told the court.
He continued the attack, knocking her off the toilet seat where she was sheltering and throwing her around so her blood was left on the walls, while demanding she unlock her phone so he could see who she'd been texting.
He disputed a forensic pathologist's claim Thompson had been hit at least 70 times, saying it was more like four or five.
When his partner lost control of her bladder and bowels, he said he cleaned her up, apologised and asked her to phone the police so they could come arrest him.
"She was just moaning, telling me to shut up," he said.
When she became unresponsive Te Hiko sat next to her with a loaded shotgun under his chin.
He spoke of how he had freaked out when he hadn't been able to rouse her. He kissed her, telling her to wake up but when she didn't, he had taken a shotgun from his wardrobe, loaded it, sat beside her on their bed and prepared to pull the trigger.
"Then I remembered my nephews were coming that morning to help skin two sheep and I didn't want them to find us like that," Te Hiko said through sobs.
He described unloading the gun then cuddling Thompson to try to wake her.
He acknowledged he was unable to see her breathing or find a pulse but because she was warm he considered there was some hope she was alive.
