"Why didn't you tell me earlier?"
=> 왜 진작 말 안 했어? ( colloquial - most likely in real life )
=> 왜 미리 말하지 않았어 ?
The sentence you listed is more likely to be spoken like this:
이럴 수가... 무슨 말이야? 이걸 이제(야) 나한테 말 해? (헐) 진짜 [XXXX]. 왜 진작 말 안 했어.
The "[XXXX]" part may be:
미치겠네 / 돌겠네: drives me crazy.
기가 막히네: don't know what to say.
열통 터지네 / 열불 나네: drives me up the wall (makes me angry).
믿을 수가 없네 / 못 믿겠네: can't believe it (the weakest of these and least likely to be said)