ADVICE PLEASE!! When people are mean to me it’s hard for me to speak up for myself

Whenever a stranger or acquaintance says something shady/ out of pocket to me I just stay quiet & I hate it! I can never speak up for myself & then I end up dwelling on what I should have said to them for days 😩 it makes me feel like shit because I feel like I should be sticking up for myself. Yesterday at McDonald’s I asked for more sauce & the employee gave me the stankest look ever & threw the sauce at me & I said NOTHING, she didn’t even give me enough, I’m so tired of myself. But whenever I face confrontation with someone I don’t know my stomach sinks, my palms start sweating & my face gets hot. I feel like such a little bitch. I’m such a non confrontational person but I want to change that I’m tired of people saying what ever they want to me & getting away with it. I don’t want my son to grow up & watch people walk over me because that’s how I saw my mom let people do her.