Kokinshuu #687
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The script text is poem #687 from the Heian period Kokin Wakashuu (abbrv. Kokinshuu) Imperial poetry anthology.
Specifically, it is from part 14 of the collection, one of five parts [11-15] covering the topic "Love":
古今集 巻第十四（恋歌四） #687
飛鳥川 淵は瀬になる 世なりとも 思ひそめてん 人は忘れじ
あすかがは ふちはせになる よなりとも おもひそめてん ひとはわすれじ
A more natural translation is:
Even if the world be as inconstant as Asukagawa's depths turning into shallows,
one will never forget about the person they loved.
Or, more bluntly:
Whatever happens, I'll never forget about you, the one whom I love.History[Like] Asukagawa's
飛鳥川 (あすかがは)
Asukagawa: river in Osaka Prefecturedepths turning into shallows
淵は瀬になる (ふちはせになる)Even if the world [be this way]
世なりとも (よなりとも)*squeeze*"Specific Erase History"[Someone] who is deeply in your thoughts
思ひそめてん (おもひそめてん)can never be forgotten
人は忘れじ (ひとはわすれじ)
