In the first public statement since he died of a suspected overdose last week, Grande 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.
When the rapper was arrested shortly after for driving offences, some fans appeared to blame Grande.
She then released a note describing their relationship as "toxic" and suggested his substance abuse was to blame for the break-up.
"I have cared for him and tried to support his sobriety and prayed for his balance for years (and always will of course) but shaming/blaming women for a man's inability to keep it together is a very major problem," part of her statement had said.
Grande and her new partner comedian Pete Davidson suspended comments on their Instagram accounts shortly after Miller's death after a barrage of replies from fans again blamed them.