I swear it wasn't this hard 15 years ago 
I would chalk it up to being 
er but I can still jump into a random game of Gears of War and do okay, so who knows. I guess the skill ceiling in the old Halos is just much higher due to the battle-hardened diehard audience left playing them?
I don't think I can acclimate back to controller. It's a shame the aim assist is what it is since mouse and keyboard feel so good in Campaign and Firefight.
It's also weird to me that of the two Xbox franchises I used to play, it's Gears of War that's easier to play on mouse and keyboard when one is a standard FPS and the other a third person shooter with quirky movement mechanics revolving around animation cancels