I am a female in my thirties who has had diabetes for at least 14 years, who was on insulin in the past and recently was trying to it with oral medications, but my hemoglobin A1c was 13%, and I am back on insulin. My most recent blood tests showed a blood glucose of over 400 mg/dl [mmol/L] and a C-peptide 3.5. I have always been under the impression I had type 2 yet, based on this recent blood work, my doctor says that I have type 1? How is that possible with a C-peptide in the normal range?

You should ask your physician what they meant by that. A way to interpret the results is that you are not at the point where you have no insulin secretion. The C-peptide of around 3 is indicative of some function. However, it is not enough function to maintain normal blood sugars. The typical model of type 2 diabetes would suggest that you lose function over time.

Another way to distinguish type 1 from type 2 diabetes is to check antibody levels, especially the anti-GAD antibody. This remains positive long after the onset of the diabetes. If positive, it suggests type 1 diabetes. In the end, this is a gray area and absolutes are not always present.

JTL