Drop the “sorry” from the front of your no. You’re not in trouble.

Opening with an apology turns a normal decision into an offense. Sorry, I cannot, tells the other person there is something here to forgive, and occasionally they will take you up on that. It does something to you as well, reinforcing every time that having a limit is a small wrongdoing you need to soften first.

Try the same sentence without the front of it: I cannot make that work. Add warmth if you like, thanks for asking, hope it goes well, but keep the apology out of it. Save sorry for when you have actually done something, and hear how much steadier the no sounds.