I've been playing online FPSs for years now, but I've always been absolutely terrible at them. I can maybe get a 1:1 K:D ratio on a good round, but my average is probably closer to 1:3 or 1:4. This has been pretty consistent in every FPS I've played over the years. I'm not sure what I'm really doing wrong. I use cover. I try to flank with my teammates. I throw grenades to flush enemies out of cover. But somehow it always seems like they're faster, more accurate, better able to get the drop on me.
Is it just because I only play them for a few weeks or months before moving on? Is this something that gets better with practice, or do I have some kind of undiagnosed motor deficiency?
Try to remember that some engagements are not worth trying to finish. Sometimes it's best to just run away and either try to ambush if they follow or lose them in another part of the map. If you and an enemy are shooting at each other and you are nearly dead and you know you got some hits on them too, forget it. Assume if you haven't killed them at that point, you're not going to. 9/10 times, you're going to lose. Leave the area, let your character heal (depending on the game) and find another enemy to shoot at.