Push through the bad feelings. When you say “no. Can’t. Don’t want to, etc” you’re going to feel terrible. Don’t go back on those feelings because you have to tell yourself, “if I backtrack now, I’ll be a doormat to these people around me forever.” Be a bitch in someone’s fantasy than their mat in reality
Ask yourself if you were in someone else’s shoes, would you let yourself be treated in certain manners? Or are you letting it slide? I also think the biggest thing is confidence. You don’t have to be confident. You just have to pretend to be confident. Be firm. Be yourself. And eventually, you’ll come realize you aren’t pretending anymore.