I'm sorry I don't know you very well, so this might not apply to you. But when I've had trouble with being constantly tired in the past, I tend to get my iron levels checked. Knowing that about myself, I take vitamins every day. I've also changed where and how I do my homework. I do almost everything sitting at a table and I take notes when I'm reading to help me stay engaged and focused. If I start to have trouble focusing, I get up, walk around a bit, maybe make a cup of tea or even take a little break, and then try to get back to it. If I take a break, I try to put a specific end time on it and stick to it.