Hi,
I was diagnosed with breast cancer December 2012 and went through 12 months of surgery, chemo, radiotherapy and Herceptin infusions, no hormone therapy. At the time I wasn't diabetic ... that came later. Mine was also caught early, and at the moment after the usual 12 month review checks, am cancer-free. My experience of the treatment is that it is chemo that makes you gain weight, although I do believe the hormone therapy tablets are also culprits. I gained over a stone, but this was mainly because of the dietary changes I made to help with the chemo side effects. As you aren't having chemo, you won't have this problem, nor the steroid problems. It will be the tablets you have to watch.
You are doing so well with weight loss and BS control. I hope this doesn't spoil things for you, but as you are not having chemo you will still be able to exercise and stick to your current diet, so may not have the problems you are imagining. Not everyone puts on weight. It might just require a little more care.
Good luck with your treatment, and I hope you feel as good after it is all done as I do.