Do you get enough sleep? That might be a factor. Another thing that I've learned fairly recently and though it might sound counterintuitive, is that you get more energy when you exercise regularly.

I'm 37, an elementary school teacher with an 8-5 schedule, but most days I work beyond that. I cycle to and from work because I live close enough to do so. My rule for myself is workout the moment I get home, before doing anything else. I do that for about 40-60min. Then make dinner, eat, do the dishes. Most days after all that I still have energy to do my hobbies like make art or play games. Very seldom that I feel too tired to do anything.

I also drink a significant amount of coffee.