Hi and welcome to the forum, although it is sad to be in such circumstances. Poor you, you have really had a tough time.
I am a type 2 and not on insulin, and when I had my breast cancer I wasn't diabetic. However, I do know that chemo involves taking steroids, which do not help with sugar levels. I blame my chemo treatment and the diet I followed whilst on chemo for giving me diabetes. I'm sure if you need chemo you will receive plenty of help and support with your insulin.
My tumour was an invasive ductal, grade 3, HER2 positive. I had a lumpectomy within 2 weeks of diagnosis, followed by 6 sessions of chemo then 3 weeks of radiotherapy. I also had to have Herceptin infusions once every 3 weeks for 12 months due to the HER2 positive element. I am due my second yearly mammogram later this month. So far so good, and all clear up to now.
Good luck with everything, and my very best wishes. If you want to discuss anything, please feel free to send me a private message.