Struggling with Energy Levels Due to Vitamin Deficiencies

Hello, I’m a 19 y/o with EDS and POTS confirmed as well as vitamin deficiencies and low ferritin. I noticed a while ago that I get super mentally exhausted by the afternoon and struggle through schoolwork and homework done after that while my peers can just work away with small amounts of exhaustion. Then, I started taking a Vitamin B complex and my energy stays nicely consistent throughout the day, it has many vitamins and I take it daily. Not sure why, but when I don’t take it I’m back to exhaustion and can’t do schoolwork (which is really debilitating being a college student) for very long. I have to lay down and watch something to feel better. Anyone have similar experiences? I already take some vitamins and supplements for my deficiencies in Vitamin D and iron (+ vitamin c to help with absorption) all directed by my doctor who specializes in EDS. Maybe I’m missing or lacking some other vitamin in the complex, but I am unsure as to why I’d take iron, but still need the vitamin b complex to get me on my feet by afternoon and not run out of energy (despite getting good food all day). I will probably need an iron infusion, but I doubt thats causing the during the day issues and more causing my sleep issues.