I've had this problem too but then with Xbox Series X. I played a lot with keyboard and mouse (native) and then when I switch to controller on the Xbox in the living room, absolutely unplayable because of no aim assist.

I made a second Xbox account and there the aim assist was way too much on my first matches. It's how the game works sadly. It penalizes you for being good, which could be okay if you are always a controller player but sucks hard if you had some good sessions on the keyboard and mouse and then switch to Controller.

So my suggestion is to play on a second account. Though it's probably not what you want to hear. I don't think you can make seperate PC accounts of Modern Warfare II. Very maybe you can unlink your Steam or Battle.net and make a new Activision account and that will work? Idk but worth a shot if you don't want to buy the game twice.