I’m looking for an idiom that means: There will come a time in everyone’s life when they tumble down, or experience a hard time. Something you’d say to a smug but successful person who laughs at your misery.
It sounds like you’re asking two different things. There’s a general expression to indicate that everyone experiences some misery in their lives, and then there’s an expression you’d use as a comeback to the person you mention above.
As a comeback:
What goes up, must come down.
As a general philosophy:
Into every life some rain must fall.