As far as I can remember, Gatsby went on to serve as a soldier. During that time, he attempted to get himself killed multiple times and always failed.
An appropriate word is mistake
Explanation:
A non-restrictive clause is a clause that is not needed in order to identify the word it modifies.
Ex. Wh00t, a brainly user, went to the store to buy some carrots.
So "a brainly user" is additional information about "Wh00t" that is not necessary to identify "Wh00t," therefore it is a non-restrictive clause.