I am not very trained in Shooters, but some of the random people i play against seem to be über-humans with über-skills and reflexes.

i mean people give me instant headshots all over the whole map while i am sprinting in stealthmode?!
i hide behind a piece of wall, i am in stealth mode and only my head shows over the wall, but people come around the corner and give me headshots like i am wearing a neon-sign at my head.
First i thought they use thermo-vision, but kill-cam shows nothing like that.
Stealth-mode seems to be absolute senseless. everybody sees you anyway and everyone has this über-radar kill-streak ****. Sometimes i think this is getting rediculous. i mean i see nothing and they see and hit everything?!
This kill-streak-gadgets are noob destroyers like in CoD. "Pros" get everything and noobs just feed them without a real chance.
I don't even know if i am frustrated because i am bad or because they are cheating with aimbots or make-stealh-mode-red-shining-mods.

Cloak glows when grenades explode, making you completely visible for anyone to headshot you if he has good aim.

You'll develop the skill in time if you have patience. Watch good tutorials on youtube for example. Knowledge comes first, then training. All players experience what you describe and some of them become veterans in time...

I am not very trained in Shooters

that explains all of your problems

I am not very trained in Shooters

that explains all of your problems

Why so harsh, huh?

What i wanted to say is, that i never played intensive. In the past i played a bit CoD: Black Ops in MP, a bit BF3, a bit BF2142, a bit Planetside 2 and other things like that. Never longer than a month or two, never in Clans, never with friends (because none of them play the games i play, they prefer Dota or LoL). I was always average, not bad and not good. But here it's like i am playing in the ESL, perma owned by everyone else. What happened to all the other averages and noobs?
It's like they magically disappeared.

Edit: The last time i played BF3 was when Armored Kill DLC was released and i was like this:
http://battlelog.battlefield.com/bf3/soldier/Stingshot22/stats/191964788/
Not good, not bad, but average. But here ... no chance.

I'm not extremely good either but I believe your problem is that you stay still when you've cloaked. A lot of the kills I have made on cloaked players is because I have seen them cloak when they have run into cover and I expect them to be hiding there when I arrive and shoot them in the face. Always try to keep moving, never stay in one place too long, especially if someone has been shooting at you and always keep an eye on your radar. Hope this helps.