Welcome to Fluther.
You really need to define terms better. You gave some examples of the “fast food” that you’re eating, but you still didn’t give enough detail. (We can agree that french fries as “fast food” are generally unhealthy—which doesn’t mean that they’re causing you any harm if you eat them in strict moderation—and I wouldn’t say that “only once a day” is “moderate”, for example. But “in moderation” is another term that could use definition.)
It’s not how fast the food gets to you that’s a problem, it’s “what is in it?” that might be. Most “fast foods” sold at chains such as the national hamburger and taco chains, is pretty well loaded with fats, sugars and sodium that help to improve the flavor for people who are accustomed to those flavors, and that’s what makes them “unhealthy”.
On the other hand, you can find places that offer “fast foods” in the way of salads, sandwiches and the like that don’t include a lot of processed meats, sodium, sugars and saturated fats. Those will generally be healthier in the long run, assuming that you eat a balanced diet that includes your daily requirements of all nutrients. (For example, it’s considered “healthy” to eat fruits, but if all you ate was fruit you’d be lacking a lot of protein in your diet.)
And there are other causes for losing energy, such as lack of sleep, excessive use of alcohol or other drugs, dehydration and other medical causes.