File a complaint over one missed appointment? Seems a bit over the top. Perhaps your reaction is as much about your other bad experiences than this one incident?

The therapist wanted to speak to you personally and apologize which seems like a therapist who is willing to admit her error and would probably be careful not to make that mistake again, but since you terminated hopefully the next therapist will not make any errors. I doubt it though. I know you don't want to hear this, but mistakes happen. You had a therapist who was trying to accommodate your schedule and messed up, probably because it was completely out of her regular routine. It is unfortunate, but it wouldn't have been a fatal error for most people, particularly if this was the first and only time this had happened.