How to be mentally unbreakable?

And I mean this in different ways:

1) Self talk: i tend to have a weak image of myself.

2) I started doing muay thai and i want to do competitions one day so i want to control my facial expressions but also be mentally unbreakable. My opponent can say what he want but i just don’t care.

3) in real life if someone tries to manipulate me into thinking a certain way or blaming me.