I just found out my 17 year old cat has diabetes?

My 17 year old cat went to the vet today, the vet took a blood sample and told us that she has diabetes. We took her because she was peeing a really lot, and we didn't know what was wrong. We don't want to inject her with needles twice a day because she would hate it, and we don't want to put her through pain. So we are going to try and control it though diet. The vet gave us Purina DM to try. Will giving her this food help her? Will she stop peeing so much? Will she stay healthy? Can we still give her treats? I did research, but I still want to know more. How can I help her? Is it bad that we won't give her insulin shots? Thanks for the help.