How to stand up for your self?

I didn’t want to post the question because I know the answer would be to stand up for my self but it is really hard. I have terrible social anxiety and I hate saying no to people. It’s very hard for me to say no and very hard for me to face confrontation so this question is for all the other shy people out there. If you were in a situation where you had to stand up for your self how did you get the courage to do it and how did you deal with it?

My situation is my coworker/manager who is an alpha in my opinión. She is not scared of anything. She asked me to do some work that i do for our company but for her personal use... I have done it before because I’m scared to say no. She is my boss kinda...but now i don’t want to do it and I don’t know how to tell her no because I’ve seen herManipúlate before and I know she might do it to me since I’m weak.