You are past those first 12 weeks so as long as you keep it sensible, and it sounds like you are by avoiding other coffees and caffeine products, then a Big M sounds just fine to me. I drink a weak latte most week day mornings and that is about it with the occasional chocolate and coke thrown in. You will know if you are having too much, and if it is what is helping you through the day then I say go for it. If you were trying to down strong espressos then I probably would be concerned, but a milk with flavour should do you no harm. In fact it is probably the milk and taste you are craving more than the caffeine itself which is why you are off normal coffee!