I have been trying to figure out this sentence “これまでにない上出来だ” looking it up it reads as “You have outdone yourself” however I’m confused because it seems like it says, “up until now you have not out done yourself”? the main reason it reads like this to me, is because of the ない part. ない usually means its not. so why is it in this sentence? can someone please break down the idea behind this sentence?