How tired is too tired?

I feel exhausted all the time. I’m only 22 years old and I feel like I can barely function. I work as a nanny full time, and I have to get up at 5:30 am, home by 5:30 pm, but when I get home I barely have enough energy to do anything. It’s not just laziness like I don’t want to do stuff, it’s just it’s hard to do it. Tonight my BF and I got invited to play games at a friends at 6:30, and I’m dreading it because I know I will be so tired and I’m usually in bed by 8pm. Some days are better than others, but most of the time I’m like a zombie and just work and sleep as much as possible. I know this isn’t normal. I’m not sure what to do to increase my energy, or what could be making me so tired. I’m overweight, but I feel like that can’t be the whole problem because most people I know are also overweight and they live normal lives.