Matthew Perry's ex-fiancée is honoring him with fond memories -- but also, some hard truths ... this just days after his sudden passing.

Molly Hurwitz -- a literary manager who was with MP from 2018 to 2021, and briefly engaged during that last year -- posted a tribute to Matt by throwing up a silhouetted photo of him and adding a long caption ... reflecting on their "complicated" relationship.

She writes, "He would love that the world is talking about how talented he was. And, he really was very talented." Molly goes on to say she would watch "Friends" with Matthew and that he'd be proud of the work he did, pointing out certain scenes and giving himself kudos.

MH adds, "But, I obviously knew that man in a very different way, too. While I loved him deeper than I could comprehend, he was complicated, and he caused pain like I’d never known. No one in my adult life has had a more profound impact on me than Matthew Langford Perry. I have tremendous gratitude for that, for everything I learned from our relationship."

She finishes her post by mentioning Alcoholics Anonymous as an invaluable resource for people struggling with addiction, and finally says ... "Matty, I feel relief that you are at peace."

Like we said ... Molly and Matthew were set to be married in '21, but their engagement fizzled out after just over 6 months. At the time, Perry was quoted as saying -- "Sometimes things just don't work out and this is one of them. I wish Molly the best."