"I traveled for five hours to see you and you didn't show up! I don't know why I bothered." "I traveled for five hours to see you and you didn't show up! What a waste of time." "I traveled for five hours to see you and you didn't show up! I should have gone to the beach instead."

'I wish I'd stayed at home!' *I wish I had + past participle* is commonly used to express regret without sounding too formal.

I like Jason's answer. You can also say something like 'I regret coming all the way out to meet you, since you didn't show up.' Or 'I'm regretting...' There isn't really an informal expression used to Express regret, short of saying 'damn'.

I would say, 'If I had known, I would have stayed home.' You're looking for the grammatical concept of the past unreal conditional (aka Third Conditional) - a way to express regret: https://learningenglish.voanews.com/a/past-unreal-conditionals/3181755.html Past perfect in the 'if' clause (first part of your phrase): https://www.lawlessenglish.com/english-mistakes/if-i-would-have/ Don't be afraid of the complexity. We English natives don't really learn this in school (at least not in Anglophone Canada), we just become accustomed through ordinary use. Heck, this is the first time I've seen 'past unreal conditional' as a formal term, but I use it all the time in speech and writing.

"I traveled for five hours to see you and you didn't show up! (If I had known, I would have stayed at home)