What's wrong with me??

I am diamond 2. But I think that my rank is silver or even bronze, I can't kill almost anyone, I get hit in the head almost immediately as soon as I come out from behind the corner. It seems that I practice the placement of the crosshair, I pick normally, but I can almost or even be overtaken by a guy who is on iron-bronze and plays diamond, this is abnormal for me. Ping - 50-60 mc, fps - 110-120. When my teammates hit the enemy who killed me, he starts to miss with almost all bullets. You may think that I just pay attention only to these cases, but my teammates really can't be killed. As I said earlier even weak players can overtake me, so here's the truth, and this player played like on iron. As a result, I always lowtub. It's not even about positioning. I just lose my fights, and my aim seems to be good, but sometimes I can't hit at all. I will be grateful for any help (I have been playing for a very long time) (sorry for my bad English if there are any mistakes)