I don't know what distinguishes me from a diamond, thus I can't get to diamond.
I'm currently a plat 2 jungle/top/support main.
I don't know why, but I'm finding that recently either I have a great early game but then the enemy shuts me down in 5v5 teamfights, or I get shut down early and then never come back. This is obviously a problem because when I'm ahead, I can't guarantee to win, but when I'm behind, it seems to be a guaranteed loss. There's something about my play that is separating me from diamonds, but I don't know what it is.
My last 2 losses:
I start 4-1 as Master Yi. I start snowballing and take enemy mid inhib, then take Baron. It was almost like a guaranteed win until a crucial late game teamfight; I was splitpushing bot while my team was pushing top inhib tower. Our team gets engaged on, I TP in, but our team loses the fight. Every fight afterwards then I got shut down by enemy Cho'Gath and TF who permastun me despite me building semi-tanky. We lose 1 crucial fight where we get aced and enemy ADC survives with 100 HP. She takes our inhib then nexus turrets, then nexus.
I start 5-0 as Shyvana. I start snowballing and kill every lane. However every lane dies solo whenever I'm not there and Kalista feeds Vayne. Our comp has no point and click CC, thus we get wiped by Vayne.
I don't know what I'm doing wrong. I don't know how to think like a diamond.