I am by no means a master of writing letters. However, I think in this instance, I would stick to the main points and try to keep it brief. In order of importance, in the eyes of the doctor:
- The neurologist prescribed them because they deemed it necessary.
- The dosage they prescribed was also due to it being a necessity.
- The pharmacist has publicly shamed you for needing medication which has had a negative effect on your mental health.
I wouldn't mention about costing the country money, as that should have no bearing on you getting the support you need. I would be more than happy to proof-read a letter for you if you reply or message me, although I can't promise I'll be timely about it.