If you have an identified trend of low blood iron as well as ongoing fatigue, your GP really should be running more tests - full blood count, check for other nutrient levels, etc. There are a number of health issues that could be causing it, like coeliac disease or other malabsorption issues, so it is important that you get it checked out more thoroughly.

On the other hand, the iron may be borderline but not strongly indicated. Other issues that can cause the type of tiredness/fatigue/weakness that you're describing include - Vitamin D deficiency, deficiency in calcium/magnesium/potassium, chronic fatigue syndrome (which is an actual thing, despite what some doctors have thought in the past), underactive thyroid, diabetes, or even plain ol' lack of sleep or sleep disorders. I cannot stress how important the Vitamin D/magnesium/sleep connection is. And loads of people have sleep disorders without knowing it, so if you can't find any other answers and experience any kind of sleepiness with your fatigue/weakness, sleep is something you should get checked out too.