I got demoted from g5 to s5

Try not playing ranked.

?

Uhhh. Well..

i forgot i can troll in preaseaon and make my mmr bronze :D thanks for reminding me that i should troll on my main acc to get low mmr so i can stomp bronzes in ranked :d

Try playing something that is not an assassins more.

They are ABSURDLY overpicked (assassns in particular, and similar champions in general) in preseason, so even if your champion is strong and you do good on them, you are still extremely likely to compromise your team composition to be very squishy and lack CC.