I really feel for you, I experienced exactly same thing. I know how completely exhausted and defeated you must feel. Low iron is awful and debilitating when it gets this bad.
My ferritin levels hovered between 4 and 20 for 10 years after I had a bad PPH from my first's birth.
Low iron is a web - it causes and is caused by lots of different things. I was on 200mcg ferrous fumerate a day for a long time but I wasn't taking it as well as I should have been and it wasn't really making a difference. In the end an infusion really did the trick and now my levels are very high. But I did see a significant improvement on the tablets (I got my ferritin from 4 to 25) when I got better at taking them from doing the following:
I took my tablets on an empty stomach with Urgent-C, a high dose vitamin C supplement
Always took the tablets at least an hour before or after caffeine, dairy, magnesium
Took a probiotic to help my digestive system
I also started to take vitamin D, a broad Vitamin B supplement and liquid B12
Ate as much iron rich stuff as I could, lentils, chickpeas, cacao powder, cherries, dried apricots, lots of green veg obvs, a steak every now and again
I made sure I stayed on top of my hypothyroidism by taking my levothyroxine properly (hypo and low iron are linked and exacerbate each other)
Stopped drinking alcohol and reduced caffeine as I understand these leach B vitamins and iron from your organs
I went on the pill to stop my heavy periods
If you have "always" had low iron levels that is important and means something. I always had too, it was because of my heavy periods and poorly controlled hypothyroidism. You might be celiac, as another poster suggested.
Hopefully some of that will be helpful to you. Keep going and keep trying to get the GP to help you. It's such a struggle and not what you want to be doing right now I know. But if you can get something else in place then you will feel so much better. Sending lots of good wishes to you x