Wrong effective date for Supplemental claim

Hey all.  Back again.  I recently filed a supplemental claim for mental health diagnosis less than a month after my initial decision.  My initial decision for the mental health claim was other stressor related disorder even though I have had multiple diagnosis of PTSD from personal psychiatrists.  I was also rated way lower than I should have been due to the C&P examiner.  I was recently approved for a change in the condition from other stressor related disorder to PTSD and increased from 30 to 70%.  They changed the effective date of my mental health diagnosis to january 5th of 2021 (when I had my supplemental claims C&P) under the basis that it was the “earliest ascertainable date of diagnosis of PTSD” effectively screwing me out of over 15000 in back pay from the initial effective date of march of 2019.  My argument is that the C&P examiner told me that my mental health should have been rated at 70% from the get go.  He also said that I should have been awarded PTSD from the get go as it was “clear as day” I have PTSD.  To me it makes no sense that they change my effective date of an already rated mental disorder when all they did was change the diagnosis.  But they initially DID acknowledge a mental health condition and rate it as of march of 2019 (albeit a different diagnosis).  I also filed the supplemental claim way under the time limit to be actively pursuing my claim.  Attached is the excerpt of their reasoning.  What can I do?  I already filed an HLR via fax yesterday.  Is there a good chance I can get my effective date restored?