Persistent tiredness has many possible causes, including poor sleep, stress, thyroid problems, depression, infections and other nutrient issues, not just low iron. Iron deficiency is a common cause of fatigue, especially in people with heavy periods, in pregnancy, in some vegetarians, and where there is gut blood loss, but you cannot reliably diagnose it by symptoms alone.
Taking iron when your stores are already normal does not boost energy and can cause constipation, stomach upset and, with chronic excess, iron overload that damages organs. Some people also have conditions where extra iron is dangerous.
The right step is a blood test to check iron levels and the reason behind any deficiency. If supplements are needed, a doctor or pharmacist can advise the appropriate type and how to take it for best absorption and fewest side effects. Seek prompt medical review for tiredness with weight loss, breathlessness, or blood in the stool.