This doesn't read like a lyric to me. Maybe my head is in the wrong place this morning (yes, it's morning here where I am), but it feels like free verse. There's no rhyming scheme, no hook, no verse/chorus structure, and no rhythm that I can discern. Like I said, it might be just me, but I can't figure out a way to make this into a song.
That being said, this piece paints vivid word images in my mind. I have no trouble following the story of it as poetry. I glean strong feelings of anxiety, depression, yearning, and nostalgia.
Keeping the idea of it being free verse in mind, I have a couple of suggestions I think would make it a little stronger.
I’m standing on the edge of the cliff that lives in my mind
with thoughts of yesterday and of my decline
And...
the trips that we took to seaside with my brother
go further and further away every day
it’s not sadness that I feel but longing
In my opinion unnecessary instances of the word "that" weaken poetry (and lyrics and novels and short stories...)
even when I dance
my feet are splashing splash in yesterday’s water
I'm recommending staying in present tense all the way through instead of using past tense in this one line.
I know that nothing can last endures forever
I’d just like one last time for my goodbyes
Using "endures" keeps you from having the word "last" twice in a row, plus endures gives a feeling of more effort
That's what I noticed and, as I said, it's just my opinion. Use it if it works for you or not, if it doesn't.
Nice one.
Vicki