I actually have a theory that they are using a story in halo evelotions, Called “human weakness” the story is basically what happens to cortana between halo 2 and 3, and in that story the gravemind is talking to her and trying to get info out of her using the “logic plague” what if he actually was planing a sequence of events from before everything a plan b, in a way that would have Cortona question why she was built and why halsy made her to die, things like these where all things the gravemind said, think about it even her idea of bring everyone together is what the gravemind said in that story, now let’s look at halo 5 cortana has all of these Guardians that can turn of all technology and destroy communications she has a halo ring the only thing that can destroy the flood and the only other thing that can stop her now is the infinity, now this is the perfect set up for the flood they can take planets and the planet itself wouldn’t even know until it is to late, let alone other planets, there is also a halo ring with no human’s or anything defending it where the gravemind can live and thrive.
that’s why I think they are using the novel the stuff the grave mind sed to her had how many years for her to think about and the fact that in the same novel they basically say for AIs 1 second for a person is an eternity for them, now this might be an examination but let’s say they process and s understand everything in milliseconds witch is lowballing it because a normal sparten 2 under combat conditions sees in 3.6 milliseconds compared to are 263 milliseconds, but what ever, she would see every year 365 days the equivalent for a normal human experiencing that same amount of time to feel like her would be 119,580,228 days or 327,617 years, she was left to only think while she was with John after the events of Halo 3 for 4 years or 1,310,468 human years now think about that having over 10000000 years to think and someone who was literally -Yoink!- ing* your mind a couple months before is something that will keep poping up in your mind, eventually you might think of them true but if it was the “embodiment of all your sins” you have but one choice protect your mind by not admitting he was right but also claiming those thoughts as your own, this is where I think cortana started to think these things
what do you think about my theory