How could I forget my Mom’s birthday?

(Chapter 3 Memory Problems in my book)

I'm not sure when this happened but I know that I actually forgot my Mom's birthday. I think that it happened quite a few years ago. She called me a few days after her birthday and she told me that I hadn't sent her a card or called her for her birthday. She asked me if I was okay. I hadn't even realized that I missed her birthday. I felt so terrible and I don't think it has happened again. I write birthdays, anniversaries in a pocket calendar in my purse and each month I transfer all the information onto my wipe-off calendar on my refrigerator. Most of the time, it helps keep me on task and I don't forget too much. I have to write all these dates down because I can no longer rely on my memory to have accurate information for me.