For starters, you say nothing works for you. What are you doing? What weapon sare you using? How are you aiming? What is your play style? Whats your state of mind when you play?
Also, maybe explain more specific types of advice youd like
My default response, stop worrying about kills and focus on not dying and slowing down your firing to focus on aim then ahoot, not shoot then aim. Sounds odd i know but my accuracy is generally decently high, and i tend to stick with ar most of the time. I just focus on aiming and defensive playstyle.
Also, your in diamond now, so youll get people that go in as teams, and people that generally are pretty adept with the game. When i play against gold its not even remotely the same as diamond (currently diamond myself)