Thyroid issues are REALLY common post-partum, so that could contribute to fatigue and sluggishness. But honestly, I would worry less about diet and make sure you have any support that you need. You are probably not feeling great due to lack of sleep and having a lot of your energy go towards a rapidly growing being :). If your oldest child is old enough start giving them chores and teaching them little things that would help you out, see if they can have a good rest/nap at some point during the day if they don’t still (or if they are even older make it a quiet reading time) and try your best to get a nap in. If you have family around ask for help, if that is hard for you find a way to make it easier (I know this is easier said than done, I have so many issues asking for help).