He said the attack was interrupted after Sidaway told him to stop and screamed, and a neighbour shouted.
Neighbour Susan Edwards heard “high-pitched screaming” and the “bushes shaking” before McKenna ran away.
Sidaway phoned for an ambulance and said: “I’ve been stabbed by my ex-partner… he’s been stalking me.”
McKenna dumped his rucksack and knife at an allotment near the park after eating yoghurt.
He then walked three miles (4km) to Bronwydd village along the River Gwili.
“I found some shelter, a quiet woodland away from the road,” he said.
“I was devastated and kept having flashbacks.”
He was there for three nights before getting a taxi and bus to Bronglais Hospital in Aberystwyth, Ceredigion.
Police bodycam footage showed Sidaway sitting on her doorstep with paramedics, covered in blood.
There were 17 areas of damage to her coat and her clothing was extensively blood-stained. She received injuries to her chest, arm and hand.
Sentencing McKenna, Thomas said: “You had been in a relationship with a woman of whose vulnerabilities you would have been fully aware.
“When she decided, as was entirely her prerogative, that she no longer wanted to be in a relationship with you, you reacted, as you had done in the past, immaturely, and more significantly with violence.
“You stalked her in the park in Carmarthen that morning, then you returned in the afternoon with exactly the same intention. It was planned, and it was wholly calculated.
“You stabbed her no fewer than 11 times. Three of those being to the chest and therefore heart area.
“That lady only survived because it was a cold day and she was wearing five layers under a thick coat. But after you did that, you left her to die in bushes in a park.
“You only handed yourself in when you developed frostbite from sleeping rough.”
He said McKenna was “full of self-pity”.