Saying No

Is it me or do you have trouble saying no? Because I lived with my Mother as an adult I see the need to say yes more than to say no.

An acquaintance of mine in my building asked me to take her to the train in downtown Philly this morning. She was not feeling well so she cancelled her trip. I made other plans for the day.  As I was walking out of my apartment she called and wanted me to take her to urgent care.  I had to say no as I was on my way to visit a sick friend.  She has family close by .  I still feel guilty.