My recommendation for you would be to slow down the process. If your instinct is to automatically say yes, just don't say anything for a second. It's okay if the moment feels awkward. It's not a weird thing to stop for a moment and think. You can even say "I need a moment to think about that." when someone throws you a question or recommends a course of action that you aren't sure how you feel about.
If those options fail, and you still reflexively say yes, you get to change your mind! You can call back and say "I need to change the time for an appointment." You can text your friend and say "Actually, I decided I don't want to see that slasher movie, sorry." You are allowed to speak up after the fact! That is just as legitimate! If you can't access your feelings in the heat of the moment, give yourself some time and space, and then do what you wanna do.