On time I broke a promise

One time I had to break a serious promise was when I told my cousin I would go prom dress shopping with her and her aunt. I really wanted to go but I could not because my other cousin also had a baby shower that same day. My mom told me that it was more important to go to the baby shower than prom dress shopping so therefore I could not go with her! I felt so bad but I could not change my moms mind. At first Morgan was angry at me but she realized we could not change my moms mind. We decided to go shopping a different day so I could go to both. I know that breaking a promise is wrong because it can lead to hurt feelings but but I could not do anything about it. My mom apologized to my cousin because it was sort if her fault but now everything is good.