I’m overwhelmed by saying yes to everyone-how do you set boundaries mindfully?

I keep getting swamped because I say yes to every favor or plan, and it’s like my life’s a cluttered desk I can’t organize. Work, friends, family-they all ask for my time, and I get so nervous about letting people down that I agree, even when I’m stretched thin. It’s left me drained, with no energy for myself, and I’m starting to resent it. I’ve read about mindfulness helping with boundaries, but I don’t know where to start without feeling guilty or rude. I tried saying no to a coworker’s project last week, but I panicked and backtracked, which made it worse. How do you guys set boundaries without the anxiety? Are there mindfulness tricks or habits that help you say no calmly?