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?
I think people have probably mentioned all these already but to add my two cents: a monitor with a 120+ hz refresh rate helps a lot, lowering your sensitivity (I'm at 1600 dpi and 0.65 in game sensitivity on apex legends, you can convert this on various websites. Compared to a lot of pros mine is slightly low) , and practicing a playlist on kovaaks, are the three real big secrets.
Tracking and flicking are the two broad modes of shooting which boil down to two modes of mouse control. Don't be scared to practice at different sensitivities: there is a general neighborhood that you should be starting out in as a human with human parts, but practicing at especially high and low sensitivites can be great exercise. The idea of "muscle memory" doesn't matter that much when you realize that shooting at different ranges requires mastery at corresponding sensitivities. The farther someone is the smaller the target and the finer your movement needs to be, the closer they are the bigger the target and the more accurate your broad movements need to be.