In the first public statement since he died of a suspected overdose, Grande in a social media post described him as her “dearest friend” and the “kindest, sweetest soul with demons he never deserved”.
In the caption of a video, she said she was “so sorry” she hadn’t been able to fix or take his pain away.
Miller, real name Malcolm McCormick, was open about his drug abuse.
He was found dead at his home in California. The official cause of death has not yet been announced.
The video Grande posted on Friday shows the couple laughing together as he asks her to stop filming him when he is telling a story.
The pair met in 2012 and confirmed they were in a relationship in 2016.
They collaborated on a number of songs together, and performed at One Love Manchester after a concert Grande was performing at was targeted by a deadly terror attack in 2017.
When they broke up earlier this year, Grande described him “one of my best friends in the whole world” and said she would continue to respect and adore him following their separation.