You're eating too few carbohydrates! The body needs a certain amount of glucose every day but it can only make so much, so when glucose is restricted in the diet the metabolism slows down to use less glucose. Generally somewhere between 50g-100g will prevent this problem. What exactly is the rationale of zero carb anyway? Just focus on health in all of its forms and you will be fine.
Sweet potatoes man. : ). Give some starch a try. It couldn't hurt. As long as you choose safe starches you can keep the benefits of paleo without low energy. Some people are fine on energy with lc. I think you have found that you are not.
As others have suggested, try incorporating some carbs you are comfortable with. It sounds from your responses that you might be a little overly focused on your food. Don't stress about trying some new things in small amounts and trying an array of them, be it sweet potatoes, white rice, or whatever. Many of us on this forum get a little too focused day to day. I would suggest stop counting and just eat what you feel comfortable eating, making sure to include some "safe carbs" every day.