I agree! I think most of us would think of the song "Big Yellow Taxi":

Don't it always seem to go

That you don't know what you've got

Till it's gone

https://www.youtube.com/watch?v=xWwUJH70ubM

Just a correction and some notes:

You only know (that) what you had was special once you've lost it!

"What you had" = "it", so there's no need to repeat the subject. I used present perfect at the end to show "the present as a result of the past", although present simple is not wrong.

"To lose it" can also mean "to lose your temper", so be aware there's a double meaning to that phrase.

There's a saying, I think it's a U.S. saying but it might be common to all English-speaking countries:

"You never miss the water till the well runs dry."

I found it in a 1908 book published in the United States but it might be much older.

Great saying:)