If you are feeling constant, ongoing fatigue and weakness, you may be suffering from anemia caused by a deficiency of iron. Iron is found in a lot of meats, especially in liver. It is also found in beans and some legumes.
Suffering from anemia will severely impact your ability to accomplish day to day tasks as the body fights to just be able to do basic activities. If you suspect that you are suffering from anemia, you can find out by getting a blood test. If it turns out that anemia is the cause of your weakness and fatigue, then usually an iron supplement can take care of the problem.