Welcome aboard, you don't have to choose, you are allowed to have both depression and diabetes :? Having more than one diagnosis often means that some things might not be picked up properly
For readings like those you probably have had diabetes for a while. Metformin is a relatively safe and effective medication, but it isn't the only thing you need to do to improve you health.
Diabetics who do well reduce the amount of carbohydrate they eat, whatever dietary approach they adopt. I started by cutting out added sugar to anything, then I looked for diet I could stick to and thought the low GI or glycaemic load (GL) was the way to go, because it looked like my existsing diet could be tweeked. Then I relaised that the poeple who's control seemd to be the best were the low carbers, that is when I cut out bread, pasta, rice and potato.
As my blood sugar numbers began to fall I started to feel better. The fatigue I'd had for years began to improve and I had more energy, the headaches and fuzzyheadedness I was getting several times a week reduced, the indigestion and acid reflux I got had gone and the constant hunger abated. As my blood sugar reduced and stabilised I felt a lot better. I am currently working on losng half my bodyweight and achieving normal blood sugars as that is what reduces my risk of developing diabetic complications.
All those things can leave you feeling prety low and depression is a difficult thing to deal with as it can also leave you feeling tired and lethargic, eating for comfort and all kinds of digestive distress! There is a lot you can do to help yourself to deal with the diabetic symptoms, which may make it easier to deal with the depression.
Take a look around, especially in the food/diet forums. Ask questions, tell us about what you eat and we may be able to suggest some changes you can make that would improve things further. You need to find a diet that you can stick too, remembering that making massive changes all at once might be too much of a shock to the system, especially if you are dealing with depresion at the same time, and may not be something you can stick to in the longer term. You need a diet you can stick to for the rest of your life, which is how long you will have the diabetes.