Why would he use the term “my mission”?
Comey's discussion came after Comey, Clapper, and Brennan held a meeting with Trump. The purpose was to convince Trump that he needed to have daily presidential Intelligence briefings with these "Big Three" so they could have more opportunity to influence him. Trump had already said that he didn't need to do this every day.
The three had conspired together to add a private discussion after this meeting between only Trump and Comey to tell Trump only about the "pee tape". This discussion would then be leaked to the press (CNN ran with it) as an excuse to get the whole" dossier" published. It worked. Comey's "mission" was to perform this task.
I'm surprised, though, that he described it as such.