Dear Felix
You must have forgotten our [DATE & TIME, ex. Wednesday, 2:30 PM] meeting. Although I understand how busy your schedule is, you have missed [NUMBER, ex. a total of three] meetings. Surely you can see that this is disagreeable and upsetting to me. Although I have tried to work around your schedules in the past, I am afraid I can no longer afford the risk of another unannounced cancellation. Regrettably, I will no longer be able to see you in the event of another missed appointment.
Regards
Lydia