A difficult choice

The manager did not like me leaving work by 6 pm.
I wanted to be on time for my appointment to donate at the blood bank.
I feel dissatisfied if I cannot do things like that because of work committments, yet I want to also be an effective team member at work.
This time I chose to keep my appointment. I know that I miss out on some overtime at work, and that extra money would be useful. I also know that the manager feels let down, having to do several tasks that evening without my assistance.
Yet I feel unsatisfied if I continue to not do at least some of the things that are important to me.