This is why I have had to have a LOT OF THERAPY to understand “you don’t owe anyone an explanation.” “No” is a complete sentence. You don’t have to justify your rationale. Do you need to break up with someone? “I don’t want to see you anymore” is sufficient. Do you need to take time off from work? There is no need to say “I have a doctor’s appointment” or “I need a mental health day”. “I need the 23rd off” is sufficient. “Sorry, no” is a good enough answer for someone asking if you can babysit. Deal with malignant narcissists long enough and you’ll start to set boundaries reflexively.