I'm always crouching and going slow but I still get owned. I guess my reflexes suck
CoD isn't a reflexes game, really. I have an easier time gunning kids down in CoD than I do in Halo -
that's a reflexes game.
Don't crouch, just don't rush. Pick an area of the map and lock it down. Don't camp a corner in a room, pick an area, say, one side of B in Dom, or one field of view near a bomb spot in Demo, or one part of the map in TDM/KC, and hold that area.
If the spawns flip, change positions and set up shop in a similar area on the other side of the map.
Start learning where enemies attack from so you can prepare for them.
Don't run out in the middle of the map. Don't rush the enemy spawn. Don't stand in obvious sniper lanes or silhouetted in windows. Don't kill three people from behind one rock and stay there after they respawn.
As much as the meth addicted 13 year old SMG rushers might make you think otherwise, rushing is
not easy, and is
not the path to a healthy k/d, scorestreak spree, or good time in general (though you will piss off your entire team by constantly flipping the spawns, thanks meth addicts).
Rushing (well) is hard, and the strongest skill in CoD for rushing or camping is situational awareness - and you can learn how to be aware more easily by starting defensively and then putting what you've learned to use while rushing.
If you're having more fundamental problems like basic gunskill, then yeah, you just need practice, but shooting mans in CoD isn't very hard, it's more about being in the right place at the right time with the right loadout.
Slow down, realize you don't need to be sprinting everywhere to contribute, think about what you're doing and what is getting you killed.
Start from there and you'll improve.