In the UK 'I wish I'd never asked' is said when the answer to a previous question is too long, too complaining or if the person answering is rude or dismissive. Eg A "Are you going to London?" B "Yes, and I am really angry about it as I hate London!" A: "I wish I'd never asked......." I expect there is a Chinese idiom that matches but I don't know what it is.

hi, upstairs friend, you well explained the context. so, here i try to translate your example into chinese. A 你要去伦敦吗？ B 是呀，不过我真的很不高兴，因为我讨厌伦敦！ A 啊，就当我没问过好了。。。（or, 啊，我真不该问你这个问题。。。there shoud be some other ways to express the same idea, but i believe these two responses make your speech sound very natural)

I wish I'd never asked (that question)！-- 我希望我从来没有问过！我（要是）不问（的话）就好了！（如果）我没问（过）（的话）就好了！我（真）希望我没问过那个问题！我（真）希望自己没问过那个问题！......sooooo many ways to say. you can omit the words in the brackets as you like. wanna speak mandarin like a native? you can do it!

我没有问过就好了. there are some ways to express that...all the answers are correct...

我后悔我问这个问题