The One Direction star penned Katelyn Cassidy a note saying: "Me and Kate to marry within a year/ engaged & together forever."
Kate, 25, left Liam in Buenos Aires and returned to their Florida home two days before his death.
Liam, 31, fell from the third floor of the CasaSur Hotel last Wednesday.
He had been staying in the Argentine capital as he renewed his US visa.
Influencer Kate, who had been dating Liam since October 2022, said: "I don't even know where to begin. My heart is shattered in ways I can't put into words."
She added: "A few weeks ago, we sat outside on a beautiful evening manifesting our lives together. I keep your note close... I know we'll be together forever, but not in the way we had planned.
"You'll always be with me. I've gained a guardian angel."
Describing her struggle to come to terms with Liam's death, Kate said: "None of this feels real, and I can't wrap my head around this new reality.
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