Where do people get their energy from?

I struggle to do more than one thing that requires me to go outside a day. I become so tired afterwards and my focus deteriorates. If it’s something that doesn’t require interaction with others then I can do maybe 1-3 things in a day, but if it involves people it’s limited to one. And, it’s not even super exhausting things. It’s showing up for one class and then I have another one in two hours (trying to work myself up for that second class is frustratingly hard), or work, or needing to go shopping, even just visiting my family or hanging with friends, and then that’s all I can do for a day. Let alone needing to study on a day where I need to do something else. I’ve tried just forcing myself but it’s hardly productive for when it comes to learning. Otherwise I just feel so done with being out and about, my mood rapidly drains as I get more tired. I sleep properly, and while my eating habits could definitely be improved, I’m confident I’m getting the nutrients needed. How do I get more energy to actually function like a proper adult?