One tip in finding that perfect quote for your dear beloved is to get inspiration from songs you both love, or a musical piece that means something to you like a song played on your first date, wedding, or engagement party.
If you aren't both into music though, look through lines from classic ballads for the perfect way to profess your love and what the relationship means for you.
One perfect wholesome source would be the songs from the 70s and 80s (a much simpler time), which have lyrics that describe love at its purest. Just take a look at this line from The Carpenters:
“I can take all the madness the world has to give. But I won't last a day without you!”
-I won’t last a day without you from the Carpenters