OK. You have quite a bit of weight to lose to get your BMI down to a reasonable level. That in itself will help.
it could well be your diet that is making you tired if you aren't getting the right amount of nutrients. You could consider reducing the carbs as much as you can, and increasing the fats to make up for the lost calories (energy).
If I were you I would ditch the Cornflakes. They are mostly sugar and very high in carbohydrate. Presumably you have milk with the Cornflakes? That is also full of sugar in the form of lactose, but you can buy lactose free milk. Try a natural yogurt with some berries thrown in.
Drinking more water is good. As much as you can manage. There is no reason to eat low fat unless you have another problem. You can eat any sort of meat, fish, eggs, butter, cream, natural yogurt, cheese. Eating more natural good fats won't make you put weight on - that is a fallacy.
Are you self testing?