This happens with every new champion i start to play with

I have an extravagant first game, going something like 14/5/7, then the games after that are just like 2/7/1 or something of a really crappy game. I honestly have no idea how this happens every time, and i try to get out of the slump with that champ, but it never works and i get raged at and then i will eventually go on tilt. Maybe it's just pure beginners' luck, and then i get hit with hard reality, idk.