Burst somebody’s bubble
Definition 1 of 1
Pro Tip 1/3Emotional Impact
This phrase may cause disappointment or make someone feel disillusioned when their beliefs are challenged. 
I hate to burst your bubble, but Santa isn't real.
Pro Tip 2/3Informal Setting
Best used in casual conversations rather than formal or professional environments. 
Sorry to burst your bubble, but your favorite band isn't coming to our town.
Pro Tip 3/3Metaphorical Usage
The phrase is metaphorical, likening someone's beliefs to a delicate bubble that can easily burst. 
I don't want to burst your bubble, but there's no way we'll finish this by tonight.