To burst someone's bubble is to spoil a happy illusion. Sorry to burst someone's bubble.
It is nearly always apologised for in the same breath, since the truth is being delivered as a kindness. What bursts is a belief that was pleasant rather than a plan that was sound.
1phraseburst someone's bubbleTo spoil somebody's happy illusion.
“I hate to burst someone's bubble but the tickets sold out yesterday.”
burst someone's bubble is a phrase.
Practised in Idioms