Neil Patterson, 55, of Horsham, West Sussex, was jailed for four-and-a-half months suspended for two years.
Exeter Crown Court heard he repeatedly threatened to kill Samantha Millson.
Charges of assaulting his ex-partner, Katherine Hooper, were dropped after she fell 80ft (24m) from Haytor.
Exeter Crown Court heard that when Ms Millson told Patterson their relationship was over he grabbed a 9in (23cm) kitchen knife and thrust it towards her.
"You and I are going to die tonight," he told her and then stabbed the knife into his leg and withdrew it covered in blood.
The court heard from a probation worker that Patterson was deemed to be "low risk".
But Jacinta Wainwright of domestic abuse charity Stop Abuse For Everyone told the court: "I find it extremely concerning that the view is that someone who would use a knife as a weapon and threaten to harm them is low risk.
"It's well known that when someone is behaving in that way it's a very high risk situation."
Three years ago Patterson faced an assault charge against his then partner Katherine Hooper, 24, which he denied.
Ms Hooper later jumped off Haytor on Dartmoor with the couple's eldest son Joshua on her back. A coroner said she took her own life.
Both died and charges against Patterson were dropped by prosecutors.
Former Det Sgt Andy James, who led the investigation into the deaths, said: "It's an opportunity lost. The accused should go there and face that accuser and if for whatever reason that is thwarted that is disappointing.
"I would have liked to have seen Kat in court. I think she would have been robust, honest and truthful."
Patterson was also handed a restraining order protecting Ms Millson.

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Richard and Peter Chambers, Rob Williams and Chris Bartley won Britain's third rowing medal, with defending champions Denmark taking bronze and Australia fourth.
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The Chambers duo became the first British brothers to win an Olympic medal since Greg and Jonny Searle took bronze in the coxless four at the 1996 Atlanta Olympics.
Richard Chambers said: "That was really, really brutal. We were just fighting, fighting through the whole thing to get ourselves back into contention.
"We struggled to keep with the pace of the other crews in the first quarter of the race but we dug our heels in and fought really hard."
Peter Chambers said: "We are gutted. We were unlucky to get a silver with those conditions. We're delighted with silver but we wanted to get gold.
"Fair play to the South Africans, they won that fair and square."
Williams said: "We wanted to win. We have to be happy to get a medal at our home Olympics. It's a shame we didn't win but what can you do? It's a shame."
The British quartet, who took bronze at last year's World Championships, arrived having won at the
They followed up an  where they overhauled Australia in the final 250m, with the fastest time in the semi-finals to progress to the final.
They were slow out of the blocks and were down in fifth as Denmark took an early lead at 500m.
The British boat fought back and were close to level with Denmark and a resurgent South African crew as they entered the final 500m.
But they just missed out on the final push, getting pipped on the line by South Africa.
It was the first Olympic rowing gold for South Africa and the first medal of any kind for the Chambers brothers, Williams and Bartley.
The ex-partner of a mother who jumped to her death from a Dartmoor tor with her five-year-old son has been spared jail after admitting threatening a subsequent girlfriend with a knife.
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Great Britain's lightweight men's four won silver as South Africa beat them to gold by just a quarter of a second in a thrilling final at Eton Dorney.