Lyric discussion by splitups0ul 

Cover art for Let You Down lyrics by Three Days Grace

"Never want to come down"

and

"I will let you down I'll let you down"

Are the key lyrics here, I think it's about drugs. When you finally trust them, you get withdrawal symptoms. Or believe in your dealer he will start selling you the shit stuff.

I could be wrong though - could be about someone who is worried about letting someone down but the music supports my theory about wanting to let someone down and see their disappointment.