I am have trouble understanding this verse そんなダサいこと もうしたないのよ goodbye
The context is
失って初めて気がつくなんて
そんなダサいこと もうしたないのよ goodbye
My translation:
Line 1: The is bad-thing [なんて] it is the first time [初めて] I notice [気がつく] I lost you [失って]
Line 2: Lame thing [ダサいこと] like this [そんな] I already did [もうした] ... ないのよ [will not do]
I am having difficulty in understanding why ない comes after a verb in the past tense [した]. Shouldn't it be grammatically [したなかった] ? Or is this new slang ?