Practise a response that protects your own choice while still respecting the other person.
Pick one example that feels realistic. You can also describe your own.
Complete the three parts. Your response will build automatically below.
Read your response as if you were saying it to another person. Where does it sit on the scale?
A strong assertive response usually does these three things.
Try making the choice or boundary more direct. You do not need to apologise for having a different decision.
Your choice can be clear while the tone stays respectful. It does not need to sound perfect — only realistic enough to use.
Keep the boundary, but see whether you can remove blame or attack from the wording.