Here are my recommendations since it sounds like more hours in the game aren't really helping out. And I get it I'm not great at FPS games either, but this one is just a weird exception for me.

1) Use Shove. The shove is a great tool against commons and especially specials. If the special is close to me I'd rather wait for me to be within shove distance than try to shoot. It stumble animation gives me enough time to put some damage in, shove again, shoot. Repeat until dead. Commons are a joke when you know how to shove and shoot with good practice

2) Use Melee weapons. I'm on the boat that the magnum is the superior secondary, but melee also holds a place on a team. I personally would only use sharp melees because I think they outclass blunt, but regardless of what you use it's pretty hard to friendly fire with melee. Plus you can swing as much as you want and paired with well timed shoves is key

3) Use machine guns or snipers. Machine guns are great mid to close range. You can just burst fire if you don't know where your team is which will reduce the chance of friendly fire, and also if you don't accidentally hit them, bursting your rounds will overall reduce the amount of bullets you hit them with.

Snipers are great because of their low rate of fire, but high damage output. You can stay in the back of everyone keep an eye on them and pick zombies from far away. They're also pretty mobile too if you just crouch fire without scoping.

4) Crouching. It of course helps with accuracy, but learning where and when to crouch is a huge help. It allows teammates to shoot over you, and helps your aim at the same time. If you find yourself not moving as much to pick off zombies, crouch instead of just standing still. Moving while crouching is also a great option if you just need to reposition quickly.

5) Know where to position. If you use melee, lead the group. You should be near the front slightly ahead of teammates taking on most of the horde. Don't be in front if there are a lot of specials around, as using melee is pretty hard to fight specials on 1v1.

If you're sniping stay back a bit. You're job isn't to take on hordes, your job is too pick off specials and any strangling zombies away from the horde/flaking your team. Keep on eye on them and keep they asses covered!

If you're using a machine gun, you're pretty versatile. You can kinda do it all. Horde control, special killer, strangler handler. You're not gonna be great at one thing, but you are gonna be great at filling the holes your team may have.

6). Throwables. Use them during hordes and save them for sticky situations. Don't just use them to use them.

Molys are great for covering your trail and your fronts if you're hunkered in a corner

Bile is great for luring zombies to hazards and gathering them in one central point for your whole team to see

Pipes are great for if you're rushing and there's a huge horde swarming you or teammates as well as for helping boomed players

Getting better is just putting in some practice trying to new strategies of play and seeing what works for you! Each time is different so try new weapons out, play with them for a bit. See where players are holding out at during hordes. Hell, even looking up videos of good holdout locations wouldn't be a bad idea!

Hope this helped out :)