Like @JLeslie says, processed foods with lots of carbohydrates break down quickly. You will get an initial burst of energy from foods like this, and then you will crash.
For an even energy level, you should eat snacks and meals that are more balanced. If you have a bit of protein with your carbs, you will feel better. My favorite breakfast is muesli, a combination of oatmeal, yogurt, nuts raisins, and fresh fruit. A small portion of that is light and filling but delicious. Instead of eating chips or chocolate, you’d probably feel better if you ate some trail mix. I also love to make a salad with romaine lettuce, walnuts, goat cheese, dried apricots or crasins, and vinaigrette. The balance of flavors is perfect and eating it makes you feel great. Most clear soups have a pretty good balance of vegetables, meat, and starch, too. If you get bored of salads, you can make wraps filled with plenty of vegetables. (My favorite combination is grilled chicken, avocado, lettuce, bean sprouts, and dijon mustard.)
Also, stay hydrated. You can’t believe what a difference that makes.