Keith Morrison Says Matthew Perry Thought He Was Beating His Addiction

Keith Morrison says his stepson Matthew Perry thought he was overcoming his addiction before his sudden death ... but in reality, says he knew he really wasn't.

In his first interview since Matthew's passing in October, the "Dateline" correspondent -- who's married to Matt's mother, Suzanne -- recently revealed on Hoda Kotb's podcast that the beloved actor felt he was beating his addiction and in a good place ... happy, even.

Keith also acknowledged that Matt had a particularly tough struggle with alcohol and drugs -- something we all knew. He described Matthew's addiction battle as "virulent" ... noting the disease went after him relentlessly over the years.

With this in mind, Keith says he wasn't totally surprised by Matt's untimely ending. Still, he added ... "He didn't get to have his third act, and that's not fair." It's a somber conversation.

As we reported ... Mathew's autopsy noted he'd been clean for 19 months ... but his death was ruled an accident having been caused by the acute effects of ketamine.

MP had been using ketamine infusion therapy to treat his depression and anxiety -- and his last treatment came a week before his death, which TMZ broke at the time.

Matthew was found unresponsive in his jacuzzi ... drowning and coronary artery disease were later highlighted as additional factors on his official cause of death.

Keith admitted that he's still struggling with grief over Matthew's death ... but confessed his wife/Matt's mom is having a harder time with it all, even all these months later.  According to Keith, Matthew and Suzanne were closer than ever just before the actor's passing.