Give yourself time to answer
Say you must check your schedule. If she says she needs to know fast, nicely explain you can’t respond fast so she may want to find a backup. Explain that you have a pretty full schedule and need to check it. Turn them down by email. It gives you some distance from personal reactions. Turning someone down electronically is easier. After you’ve waited, say you can’t do it. Waiting helps the person get used to your not always saying “yes.” It forces them to think of alternatives to having you do what they need. Even if you say “yes,” they’ll see they can’t automatically count on you.