I want my SO to be more assertive.
What do you mean when you say you want her to be more assertive?
I mean to know what she wants, don't take shit people say
Maybe that is just not in her nature. You cannot expect her to be something that she is not capable of being.
Man, she better be assertive because if she isn't I might leave her.
You should not leave her for being herself. If you feel that way, you should not be dating her in the first place. You cannot expect her to change like that.