Vitaly Shishov’s body was found a day after he failed to return from a jog. Police have opened a murder inquiry.
“We planned a life together,” Bazhena Zholudzh told the BBC’s Newshour. “He just couldn’t leave me like this.”
Ms Zholudzh said her partner had previously voiced concerns about their safety.
However, she had shrugged them off.
“He used to sit by the window and say he saw cars coming in and out of the yard. I didn’t take it seriously,” she explained. “I said, ‘Maybe you’re paranoid. Who would be interested in us?’ But maybe he did have some sort of a premonition.”
Mr Shishov – the head of the Belarusian House in Ukraine (BHU), a group helping people who fled Belarus – was found hanged in a park in Ukraine’s capital Kyiv on Tuesday.
His death has again shone a spotlight on Belarus, which has been ruled by Alexander Lukashenko since 1994. Last year, nationwide protests over his disputed re-election were violently repressed by Belarusian security forces.
Earlier this year the Belarusian government caused an international outcry over the detention of opposition Belarusian journalist Roman Protasevich and his girlfriend.
The Ryanair plane they were travelling on was forced by fighter jets to divert and land in Belarus.
Speaking to Newshour’s Tim Franks, Ms Zholudzh said she had not seen Mr Shishov on the day of his disappearance: “I woke up and he was already gone. His running clothes had gone.”
“I haven’t been allowed to see the body to say goodbye to him because I’m not his legal wife,” she added.
Police in Ukraine are investigating whether the activist took his own life, or was murdered with his death made to look like suicide. The United Nations has said his death adds another level to “our worries about what is happening in Belarus”.
According to Ms Zholudzh, Mr Shishov had taken some precautions while living in Ukraine, such as photographing the number plates of cars and suspicious people he saw around Kyiv. He did not speak to police, she said, but he had been in contact with the Ukrainian security services.
“They told him that we should keep an eye on one another in case something happened and they might try to take somebody back to Belarus.”
She described her late boyfriend as “the best person I’ve ever met”.
“He was always cheerful,” she said. “When we came to Ukraine, he always made sure that everyone who came was safe, that they were OK. He made sure there weren’t secret police officers among the people who came.
“He always protected others. He just didn’t protect himself.”