I'm too nice to everyone, how can i stop?

i always put everyone before myself, ALWAYS. when i'm with friends i hide if i'm feeling upset or angry because i know that being miserable will just make everyone else miserable. i don't like arguing with friends and i usually just do what they want to do even if it's something i really don't wanna do. i fake smile constantly and pretend to be happy and uncaring. one of my friends recently said that i'm always uncaring about everything but i do care i just find it hard to be negative and start arguments. how can i stop being so nice to everyone without being nasty to people and bringing people down and starting arguments and losing friends??? I'M TIRED OF BEING FAKE AND BEING WALKED ON