Making sure you're getting an adequate iron and protein intake can help. A good breakfast and lots of snacks during the day should help. I've found things like nut bars and miso soup help me keep going. As well as gentle exercise and plenty of water (dehyration makes you slugish).

But there's also the basic fact that bub's still growing fast and needs the energy, so will drain you for it. So if you need to nap, then nap. Can you do your grocery shopping online?