Hello! I used to feel this way ALL the time. I did have to change my diet which wasn't easy. First off I found out i was anemic, if you don't eat a lot of meat (i don't eat anything but fish/chicken and rarely) then theres a high chance you could be anemic and if so, iron tablets will help. If it's not that (just get your blood tested at the doctors) it might be that you just have a lot of stress in your life that you don't let out and so it just stays inside and makes you feel worn out. I would say definitly drink more water and try to change one of your meals. I tried to diet and that didn't work but now I have a salad for lunch everyday with a piece of fruit. I find that pasta/bread will make you feel really lethargic. If you eat a lot of this stuff it can have that effect. It might be that you are allergic to wheat/glutton and that can make you feel like crap too. The best thing is to go to the doctors but more often than not it's lifestyle and diet that make you feel run down.
xx