How can I speak up for myself?

You can’t say no to others not because you are a good person, but because you want to be perceived as a good person by others. 
If you don’t want to be criticized, don’t speak your mind.
If you want to speak for yourself, embrace being criticized. 
You are in a contradictory state when you want to speak your mind, but don’t want to be criticized.
Embrace criticism.
You can speak for yourself anytime.