It sounds like you may have nutrient deficiencies such that you run on empty when you're eating less food. You could try getting a nutrient blood panel done to see what you're low in. I especially recommend testing for B12 (but read Chris Kresser's article about it first so you can make sure your doctor uses the right tests).
Alternatively, you could try supplementing with different things to see if any make a difference, giving each about a week or two. I've had energy improvements with vitamin D, selenium, magnesium, iodine (only after making sure I have enough selenium), methyl-B12, folate, other B vitamins (e.g. P-5-P), increasing my salt, and vitamin C.
Or you could try covering your bases and getting a general purpose multivitamin, but I haven't personally found that to be as effective.
Normally, I think people don't need supplementation on the paleo diet because it's a very nutrient dense way of eating. But if you're cutting food to lose weight and/or you have some health reason to be deficient (e.g. celiac disease), then I think it can be useful to supplement.