One of the worst things about dieting, imo. There are a few options: one is to go the stim route at least pre-wo only. You can also increase your carbs slowly until the lethargy subsides but this is not ideal, imo.
I remember Dsade saying something about this kind of lethargy being caused by depletion of liver glycogen (more-or-less stored sugar). he recommended eating a little fructose; a piece of fruit would be good. If you don't want to add any more carbs you can try sipping on BCAAs throughout the day as these can be used for glycolosis (i.e. converted into glucose). I follow CKD and this has helped keep my energy levels up when I cut calories and scale back the refeeds.
Speaking of refeeding, perhaps a leptin refeed (low-fat carb loading of simple sugars) every other week or even every week depending on how low your bodyfat is could help too. If you have been dieting for awhile, your body tries to get into homeostasis by slowing your metabolism. The refeed helps rev it back up tricking your body into thinking its back in a fed state. Not sure as to the best protocol for this last option; it depends on your body. Hopefully someone else can chime in.