You may just be naturally running out of energy, lots of people will by mid-afternoon, quite normal. Do you know that General George C. Marshall, US Army Chief of Staff during WWII, used to send his office home around 4 PM each day (at least until the war broke)? Quite different from today's Army. He said: "A man isn't worth a damn after 4 PM." One of my personal heroes. But there may be this:
Caffeine and/or sugar in the morning gives you a peak. Then you fall into a valley as they wear off. Another thing to consider is having not enough lunch to ward off the post morning "crash," or else too much to eat at lunch, making you naturally sleepy.
Try this: drink some decaf, so you'll satisfy that morning cup routine. Or if you're having 2 or more cups, try just 1 of regular. And avoid donuts & pastries, eat something without sugar. Then have a light but healthy lunch, maybe just a small sandwich with some protein & carbs, perhaps a small cup of soup, but no regular cola or any sugary & caffeinated drink.
Work these variables, and see what happens. You may feel less perky in the morning without as much caffeine & sugar, but also less droopy by 2. Your energy levels will remain more constant, with fewer peaks & valleys. And your sleep patterns will improve in many cases, too. Which is another reason you may be fading by midday, if you aren't sleeping well.