Vitamin D, 2,000iu to 3,000iu daily until March / April and then drop down to 1,000iu to 2,000iu daily until October and then back up to the higher dose once again.
I used to feel like you with cracked lips, no energy and cracked skin at the side of my nose during these colder months until I started supplementing my Vitamin D. It's one of the most important things you can take to boost your immunity to all sorts including COVID and there's compelling evidence that it can protect you from various forms of cancer.