I'm not criticizing your choice of diet, everyone needs to make their own choices. But it is also important that as patients we have access to good independent objective advice about what is the exact scientific evidence. And unfortunately in the world today filled with Dr. Oz telling the world about the latest quack thing he is selling, the world is just filled with deceptive propaganda.
In this case, Dr. Barnard has actually done a lot of the work for us on the scientific evidence. In his 2014 meta analysis of vegetarian diets he analyzed all high quality studies he could find on the use of vegetarian diets for glycemic control in diabetes. Unfortunately he was only able to find six studies to include and only four were actually randomized-controlled trials (RCTs) (gold standard). RCT's are required to make claims about the how well a proposed diet changes outcomes compared to a control and most meta analysis efforts would not include clinical trials, but he was forced to since there was so little study results to analyze.
That study found that a vegetarian diet lowered the A1cs of patients by 0.39% compared to the controls which were basically all following the Standard American Diet (SAD). No improvement in fasting glucose was identified. So the upshot is that perhaps a vegetarian diet was marginally better than the worst possible diet you could imagine, the SAD. Unfortunately the results of the entire body of evidence only showed a small effect with little statistical significance and thus we must conclude that there is a very high probability the negligible result was actually due to chance.
This is the science, but if you read what Barnard has written and hear him speak he gives you all these facts with long-words and it seems like there is lots of science. But it just isn't there. He does classic cherry picking, misinterprets studies, messes up human physiology and other things which are classic things that are done by practitioners of pseudoscience.
Make no mistake, I have read his books and heard him speak. There are some things that he recommends which I find very good and truly based on scientific evidence. And I do some of those things he suggests, avoiding sugar and eating lots of non-starchy plant based foods. But the core argument that fat (and animal fat particularly) is the root cause of diabetes and the key to reversal. I find that to be unsubstantiated. And the studies to date on the use of vegan/vegetarian diets for glycemic control have had very poor results, particularly when compared to low carb diets.