I saw a trend that truly fit wangxian so I knew what I needed to do
To make my intentions clear, it's more or less supposed to be Wei Wuxian and Lan Wangji as the time passes.
At first it's young Wei Wuxian being full of enthusiasm but not really expecting much of their relationship (definitely no being a couple haha), underestimating young Lan Wangji's feelings (and ain't that canon). And so he talks about how Lan Wangji will "forget everything that happened between them" with time.
(Slightly older than at the beginning) Lan Wangji focuses on the "you and me" part, while Wei Wuxian drills his own point home with grave certainty. He's now more distant as it's somewhere after the Burial Mounds and he probably doesn't believe he has any future with Lan Wangji.
After that, Wei Wuxian is around the time where he was in the Burial Mounds with the Wen remnants. He's determined to stay there and walk towards his own end rather than give up on his ideas ("no i won't wait for you"). He still cares about Lan Wangji of course.
"For the last time," says Lan Wangji, weeping, as Wei Wuxian is already dead.