Ariana Grande is sharing her heartbreak over Mac Miller's death for the first time, and her emotions include anger, sadness ... and deep regret she couldn't get him to stop using drugs.
Ariana posted a video she'd shot of Mac back when they were dating -- it's a touching clip of both of them giggling as he playfully asks her to stop recording. In the caption, she says, "I'm so mad, I'm so sad I don't know what to do. you were my dearest friend."
She adds, "I'm so sorry I couldn't fix or take your pain away. I really wanted to."
Mac and Ariana were a couple for about 18 months, and he was by her side during the Manchester bombing.
She insinuates they had discussed the possibility of him overdosing ... saying, "I really can't wrap my head around it, we talked about this. so many times."
As we reported, Mac's friend Shane says Ariana was extremely supportive when Mac was sobering up, and went to great lengths to keep him on that path.