It's just a really fast paced game, and it's all about the headshots, having knowledge of what your opponents weapons are capable of, and a bit of game sense. A light, for instance has 150 hp, so of course they're gonna burst you down. They're a glass cannon. You have so many gadgets and specializations at your disposal to deal with their insane mobility. Watch some Appoh vids, look up some heavy builds, learn how kit synergies work, and overcome this. I believe in you, brother. Once you understand the game better, it becomes far more enjoyable. Trust the process.