To help with the diet part perhaps have a good conversation with a dietitian nutritionist (RDN) and see what comes out of that. I’ve had to change my diet to deal with medical issues and working with a clinical dietitian has helped a lot.

Iron-deficiency can cause fatigue and a loss of endurance. Iron helps to deliver oxygen to cells, and too little has also been shown to decrease immunity. Beans, lentils, spinach, and pumpkin, sesame seeds are some good sources and eating iron rich foods with vitamin C-rich foods can also boost iron absorption. You can get the iron you need from foods instead of using supplements.

Dehydration can cause your body to conserve energy by decreasing blood circulation. That can deprive your body of oxygenated blood and can cause feelings of fatigue too.

Exercise, diet, hydration and a good night of sleep all help with energy levels. If doing that doesn’t help I’d talk to a doctor about it.