How to get better at league?

I linked my op.gg so you guys could look at my stats. Could anyone explain to me why i dropped from diamond promos to plat 4 with a 25% win rate almost all the way there? I dont see what I'm doing wrong. Even when I go negative, I'm still the leading player on my team. I just really am confused. A few weeks ago i could be even and win a game. Now, even if im 10/5/9, 10/7/13, 6/6/8, 10/5/8, or 8/6/4 I cant win. Those scores clearly dont show that i'm hyper carrying anything, but look at the rest of my teams in those games. Is there anything that I can improve? I've been looking at my detailed stats and usually I'm snowballed pretty hard in one point of the game, but I cant seem to make a lane win with me even if i kill their laner 3-4 times. Any advice?