How do I stop freezing up when people press me for an immediate answer?

I'm in my late 20s and I have this thing where if someone asks me to do something on the spot I freeze and agree, even when I do not actually want to. For example: a coworker asks if I can cover their shift next week, a friend wants help moving this weekend, or a family member invites me to something last minute. In my head I'm thinking, "I need to check my calendar," "I need time to think," or "I might not have the energy for this." But out loud I say, "Sure, I can do it." Later I end up resentful and stressed. I either push through and burn out, or I back out and feel guilty.

I have creative hobbies like writing and art that recharge me, and I keep giving up that time because I can't seem to say no in the moment. I don't want to become cold or unavailable. I just want to respond like a normal person.

How do you build the habit of buying yourself time to think before answering, especially with people you see regularly? What are some specific phrases that don't sound like excuses, and how do you handle it if they keep pressing for a yes or no right then?