Low iron or Low B12??? They're as important as each other.
Make sure you have some form of Vitamin C with your iron source - potatoes and capsicum are really good choices as it will help you to absorb iron.
B12 is available in meats, eggs, nutritional yeast and supplements. I recently switched to a different supplement, and the change has been enormous. I'm getting more power on my bike, I wake up smiling, not exhausted, and I can stay awake past 9:30.
Maybe try one of those?