When it comes to console vs PC, its an entirely different skill ceiling. Because of the natural imprecision of gamepad sticks, that auto-aim somewhat overcompensates for, there's less ability to actually put your bead EXACTLY where you want it. Pinpoint precision. Almost no-one can do it in less than 1/10th of a second. Its near impossible. Because of this, if you can't do so... You're one of the normal people. One of the good people even.
On the PC... It is VERY possible to do this, and people have been playing with the mouse for decades with high DPI to get split second headshots. The higher the accuracy of the guns, the worse it becomes. Pinpoint accurate weapons allow those with enough experience to make pinpoint accurate shots, very quickly. This leads to PC combat being over in split seconds with little chance to run and hide a lot of the time. Its a lot more twitch based than on a console.
I haven't touched a console FPS in about... 10 years. Maybe more. I hung out with some of my partner's friends, who played exclusively on console, and vsd them in CoD multiplayer. I had no idea what the controls even were. First half of the first round, I was naturally getting dominated, throwing grenades instead of shooting, fumbling with how to do anything... Second half of first round I made a comeback because I learned the controls, and had learned the map. I ended up second by 1 kill. Second round onwards I simply dominated.
I wasn't good at console FPS. Oh hell no. I'm also not good at CoD. Played it once before then at another friends house, also about 6 years ago. How did I end up defeating them? They weren't 'good' players. They were average. On the PC though, if I had never used a mouse before, they'd outclass the shit out of me. With the console taking care of aiming for me, I could concentrate on learning the maps.
If we were to play the PC version against each other sometime, I've already played other console players who were 'good' at those games and seen the results. Its utter decimation.
Being good at a game, and being good at a game on the PC are two totally different things. Don't take it to heart. Even I get pissed off at how hyper-competitive PC FPSs can be thanks to some people's aim, and I'm almost one of those people. If you want to become good on the PC with a mouse, you need to practice. It won't take care of things for you like a controller's auto-aim will. It all has to be you.
As for why you might not be good at FPS in general...
A few things. One is the maps. In any FPS knowing the map is key to winning. If you know where everyone goes to snipe, where the health kits are, the rare weapon drops, good ambush locations, central hubs that are always a shitfest of death - you're halfway to winning already. You'll survive a lot more, and you'll be able to ambush a lot easier, with better equipment.
Another is situational awareness. Don't get tunnelvision on what is infront of you. Where reasonable, always check behind you, and to your sides. Listen for the sound effects of people running or walking, and decide whether you want to engage, or hide and let them pass.
Burst fire is also something you really need, and that so many people don't appreciate. Even if your gun isn't auto-burst, emulate burst for it with your firing pattern. In the above mentioned CoD match, default guns were burst, and somehow someone changed their loadout to get a full auto. Even in a face to face firing match, I won. How? The held down fire. Their accuracy disappeared. Most of their shots missed. I pulled a quick 3 burst shots. Accuracy was not lost. Almost all shots landed. Burst is great, once you get used to it.
You also need to know the tricks of the game a little. In CoD, you can shoot through walls. Walls mean nothing. If you know someone is on the other side of a wall, shoot the wall. It'll kill them. In Battlefield, you can't shoot through walls. With explosives you can blow a lot of them up though. Even if it doesn't kill the enemy, a grenade to their wall will remove their cover.
As for failing to perform under stress... Practice. And face bots first, not other players in this case. Start 'em off on easy. Those things have pathetic skills, reaction times, and tactics. You should be able to work on calming yourself down in such situations, and then move on to better bots. Even if you can't calm yourself down, you can do what I do: Muscle memory it. I panic when something unexpected happens in an FPS. Someone jumps around a corner I thought was empty, I see a rocket flying at my Jeep, someone's pulled out an apache and I'm out in the open. My advantage is that my body knows what to do. My "Panic" is just doing what I'd do normally, as my hands are that used to dealing with such situations from bot matches and fights with friends with no stakes, that as soon as it happens, I just revert to just doing whatever my memory is for the game - which I've trained to be the right way to play. When you round a corner and panic, if your panic reaction is to just spray and pray then you're going to die. If you've trained yourself to shoot burst and how to aim without thinking about it, then you'll panic - but you'll still aim and burst as that's what you're used to doing when you're not thinking - and you'll live or get the kill far more often.
As for slow reaction time, a lot of this is situational awareness. If you react to someone before you even see them, its a lot easier to not be taken off guard.
