Moving helps. Don't know why, but it does. When I feel lassitude not related to having just done something strenuous, I get up and do something, whether it's dancing to the music on TV commercial, or well, just whatever. ....
I also now try to eat several small healthy meals per day, using my mother's and younger daughter's example.
When push comes to shove, I'll have a cup of tea. Caffeine!

I agree with eating back at least most of your exercise calories. I also think nutritional balance and even supplements has a lot to do with it. Keeping up an exercise is essential - I find it surprising how much expending energy gives you more energy.

What came to mind first was - making sure hydration is adequate. Often when I feel a slump it turns out I am thirsty.

Another thought. I discovered if I hadn't eaten before exercise or walk, tiredness set in quickly. So I eat small 30 minutes before and immediately after, choosing what to eat depending whether it's cardio or resistance. Must find the report I saw recently that advised the same thing. It was on a scientifically reliable site. If I do, I'll post the link.
Here's the link. It's from the NIH. (U.S. federal site. National Institutes of Health)
well, now I can't post the link. It is just below.

Sharon, good point about having something to eat before you workout. I like to do a smoothie with protein powder. I also avoid eating carb heavy after working out, it seems to trigger more eating and I am a slug if I have carbs after a good workout.

I agree with everything said above. Hydration is huge, so it not cutting calories too low. I will toss in two more things. While you are cutting calories are you at least taking a good multivitamin? Look hard at how many fruits and vegetables you are eating. Unbelievably, my brother was started on a statin and never told to take COQ10. When I told him statins drain it from your body and he started it, his exhaustion completely went away. When he told his cardiologist the guy said, oh yea I have heard about that! Research the side effects of every med you may be taking and don't expect that your doctor will tell you. And of course, do have a complete blood panel done!

I had a really strange day yesterday which involved eating snacks instead of lunch and ending up below my goal calories for the day. I was running around taking my sister to some medical tests, helping her grocery shop, etc. When I got home I was so tired I could barely move. So next time we have a day planned like that, more and better protein snacks are coming along!