Diddy's uttering his first words about the videotaped beating of Cassie ... saying he "takes full responsibility for his actions," and claims he's already gone to rehab.

He posted a video Sunday morning, looking directly into camera, he said, "It's so difficult to reflect on the darkest times in your life, but sometimes you gotta do that. I was f***ed up. I mean, I hit rock bottom."

He says he's not making any excuses for his behavior -- the brutal March 2016 assault on his then-girlfriend Cassie while they were staying at the InterContinental Hotel in Los Angeles. The surveillance video -- obtained by CNN -- showed Diddy chase her down a hallway, grab her by the back of the neck and hurl her to the ground .. where he kicked her and dragged her.

Diddy claims he "sought out professional help" after the incident. He says, I had to go into therapy. Go into rehab. I had to ask God for his mercy and grace."

He didn't offer any detail about how long he was in treatment.

He ended the video with an apology ... "I'm so sorry, but I'm committed to be a better man each and everyday. I'm not asking for forgiveness. I'm truly sorry."

What's interesting is he did not directly apologize to Cassie -- and, so, the video comes off more as an apology to the public than the victim.

Of course, Diddy and Cassie continued to date on and off for 2 years after the hotel attack ... so, it's likely he did apologize to her at some point during that period. They actually attended a Hollywood premiere -- for one of her movies -- just 2 days after that 2016 assault.

Remember, Cassie described the attack in the sexual assault lawsuit she filed in November against Diddy -- a lawsuit they both agreed to settle just one day later.

At the time, Diddy's attorney emphasized the settlement was in no way an admission of guilt, and Diddy himself would later deny all the allegations made against him in several lawsuits.