rain on sb's parade

rain on sb’s parade (to bring negative influence, information, or events to smb's otherwise positive experience; to do something that spoils someone's plans) — портить удовольствие; ~ обломать кайф (разг.)

Example 1: You have every right to complain about your own problems, but please do not rain on my parade.
Example 2: I'm sorry to rain on your parade but you're not allowed to have alcohol on the premises. (CALD)

Related vocabulary:
[put a damper on sth ]

[burst sb's bubble]
[обломать кайф]

If you rain on someone's parade, it means being negative or pointing out the downside about someone's positive circumstance. For example, if I was happy about getting a raise, then you remind me that it's still not enough to afford the car I want — that would be raining on my parade.

A fly in the ointment is something that "gums up the works", stops something good from happening, or ruins something good that does happen. For example, if I got a raise, but the taxes increased and I got no more actual money, the taxes would be a fly in the ointment.