Letter of apology

Dear David,
I am writing to apologise for forgetting your birthday. I feel very bad about this and I hope you aren't upset with me. I have a good memory, however, last week I was very busy with my job, and besides that, my little son, Andrew, had a cold and my wife and me had to take him to the hospital because we were very concerned/ worried about this.
Anyway, please accept my apologies for my oversight. It won´t happen again!
Best wishes,
Michael.