Bring a bag made for your camping conditions. An excellent sleeping bag is never more important than when the temperature is below zero. I once tried to use my 15 degree bag as a simple -5 degree bag. I am positive I almost died that night. Before the night was through I had used my belay parka, 3 beanies, two pairs of pants and my down boots, and still was not warm. It seemed like my sleeping bag wasn’t even warming me. Lying there, freezing rather than sleeping, I told myself never to make that mistake again. Before buying a new bag, I built a foam insert bag and a down cover to extend the range of my bag, this was a good option in a tight budget situation, but has nothing on my ultra warm bag.
Buy a fourth season tent. That mesh ultralight backpacking tent is great for going on a nice walk when it is 70 degrees. But it’s not even going to help when it is below 0. Spend the money, get a fourth season tent. Black Diamond and The North Face make some great tents. Because they are built for the wind and the cold, they are indispensable in these types of situations.
Remember you can go home. I have insisted so many times that I could survive in the
cold, even without the proper necessities. No one is going to blame you when you come
home and it is -20, but they just might if you get frostbite on your nose, or fingers and toes like I have. Trust me it is not worth saying that you are that tough. Even Shackleton didn’t want frostbite.