Hi Susan,
a. I hope you had a great time with everyone, and I promise to make it up to you soon.
b. Please forgive me for not being there. I’ve been overwhelmed with work and completely lost track of time.
c. I feel terrible about missing your birthday party last weekend.
d. It was really irresponsible of me not to let you know earlier that I couldn’t make it.
e. Let’s catch up sometime this week, Alex.