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.
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.