Multigrain is the bread you want. But instead of a sandwich (and ham and cheese or marmite is not healthy), have a:
mixed salad, low fat dressing or small amount of regular dressing, some protein like a chopped up egg, or tuna, or skinless chicken, or prawns... and a small brown roll.
Or make yourself an omelette with lots of different veg (peppers, onions, tomatoes etc).
Or homemade soup and a roll.
Or poached egg, smoked salmon and tomatoes;
Or one slice toasted multigrain bread with half a mashed avocado, chili powder and some protein (chicken, prawns etc).
Or a multi grain/pulse salad with quinoa, lentils, beetroot, walnuts, feta cheese...
There's a few ideas for you. Complex carbs, protein at every meal, loads of veg, a small amount of fruit and even smaller amount of fats like nuts and oils. Drink plenty (doesn't have to be all water, tea, even soda counts, but not too much juice).