@Aidey unfortunately, this is not unusual.
However, quite a few people have found their hospital-based diabetes team will provide a letter for free.
If it has no dates, it can be used for every trip. After about 8 years, I had to ask for a replacement because mine was almost falling apart.
In 15 years with type 1 diabetes, I have only had to show my letter once. However, I will not travel without it and risk being refused to fly or refused to take my diabetes paraphernalia with me.
For this reason, my letter lives with my passport.