Lost

[dukealien]

Once upon another time  nice twist ot "once upon a time," suggests alternate time lines, too
I believed that you were mine
Dreaming of you in my deepest sleep
Never thought you’d ever leave  leave (the speaker's) possession, that is - now known to be imaginary

You - the hope within my sight  perhaps  a more visual word than "the" here
Inside us glimmered a secret light  maybe  lose "a" ?
You - the lighthouse on my stormy sea
Yet my boat drifts - the light I’ll never see  inversion (of normal sentence structure) could use work

In basic critique, the message comes across and could be more engaging with a few edits.  For example, the near-rhymes in the first stanza are fine; you might do well to stick with them in the second since trying to rhyme on its last line led to an inversion ("the light I'll never see" instead of "I'll never see the light").

In general, rhymed poetry goes with regular meter - each line having the same rhythm and about the same length. though variations can certainly work.  When you edit, see if there are places a line could be shortened or the rhythm changed to make rhyming easy.

And, finally, look for more opportunities to use unexpected and/or more image-rich words in place of usual ones.

Hope that helps - looking forward to your edits.