Lol, this "old" gamer has the same problem. Just stick at it! You will eventually grasp it. I'm getting murdered the whole time and have to time to react. I am sure though that I will get used to it. I normally have a 0.6 k/d ratio, which is areas bad, but can live with it. Played about 6 MP games and it's far below. I will get better!
As a fellow old gamer (40 years old) who also knows without any doubt whatsoever I am not as good at MP games as I once was, just realize it is not only you.
I believe it is very simple as to why, at least in my situation...
I just do not have the time I once had to dedicate to playing the games. Less time = less familiarity with the maps, weapons, intricacies, best spots, the list goes on and on.
Sure, some people are just naturally gifted at FPS MP and they do not need time, but I believe that most people do.
Since I have realized it is not that I suck, but I do not have the time needed to dedicate to get as good as the players who are better than me, well, it is what it is. I accept it, and still have fun.
Perhaps it has also helped me that I know my K/D ratio in a video game is truly irrelevant in the grand scheme of this thing we call life. It literally has no affect on me either way. It just does not matter.
So play and enjoy. If you do not enjoy it, find something you do. Life is to short.