I know what you mean, when you want something to happen very bad but you're afraid if you say it then it won't happen (like in sports when your team is winning in the final minutes but you don't want to say 'We won! We won', because it could backfire). Unfortunately, I don't know in Cantonese how to convey this.
Here are a couple phrases I found from my Interesting Cantonese 4 book, they're close but not the true meaning you want. Sorry.
You don't want to:
口輕輕 hau2 heng1 heng1 - [light mouth] Make irresponsible remarks and promises
空口講白話 hung1 hau2 gong2 baahk6 wah6 - [empty mouth speaks blank talk] Say empty words
You want to:
講口齒 gong2 hau2 chi2 [talk mouth and teeth] Keep one's word
口密 hau2 maht6 [tight mouth] Keep your mouth shut*
守口如瓶 sau2 hau2 yueh4 pihng4 [close mouth like a bottle] staying tight-lipped
*Note on 口密, Cantodict has it listed as introverted or doesn't like to speak, maybe as an adjective. My book just says 'tight-lipped', so I don't know if it's an intentional action or just describing a quiet personality.
Good luck Donal.
Here are a couple phrases I found from my Interesting Cantonese 4 book, they're close but not the true meaning you want. Sorry.
You don't want to:
口輕輕 hau2 heng1 heng1 - [light mouth] Make irresponsible remarks and promises
空口講白話 hung1 hau2 gong2 baahk6 wah6 - [empty mouth speaks blank talk] Say empty words
You want to:
講口齒 gong2 hau2 chi2 [talk mouth and teeth] Keep one's word
口密 hau2 maht6 [tight mouth] Keep your mouth shut*
守口如瓶 sau2 hau2 yueh4 pihng4 [close mouth like a bottle] staying tight-lipped
*Note on 口密, Cantodict has it listed as introverted or doesn't like to speak, maybe as an adjective. My book just says 'tight-lipped', so I don't know if it's an intentional action or just describing a quiet personality.
Good luck Donal.