Molto freqrente il verbo frasale "Let down" significa "deludere". Vediamo il suo utilizzo:

Don’t let me down! Arrive at the party on time!

I am a success today because I had a friend who believed in me and I didn't have the heart to let him down...
Abraham Lincoln

I've never known a musician who regretted being one. Whatever deceptions life may have in store for you, music itself is not going to let you down.
Virgil Thomson

Da notare infine che esiste anche il sostantivo "letdown" che significa appunto "delusione".

The film was a letdown.

Infine chiudiamo con la segnalazione di un paio di canzoni famose (ma di canzoni che si intitolano "Don't let me down" o variazioni simili ce ne sono proprio tante...) :

• la prima, famosissima, "Don't let me down" dei Beatles;
• la seconda di David Bowie, "Don't let me down and down".

"Don't let me down" - Beatles, 1969

[Refrain] :
Don't let me down, don't let me down
Don't let me down, don't let me down

Nobody ever loved me like she does
Oh, she does, yeah, she does
And if somebody loved me like she do me
Oh, she do me, yes, she does

[Refrain] : Don't let me down...

[...]

Don't let me down and down - David Bowie, 1993

Still I keep my love for you
No place to hide no way to fall
Nowhere to lie no world so wide
I'm sick and tired of telling you

[Refrain] :
Don't let me down and down and down
Don't let me down and down and down

[...]