Lots of good info above. I have 2 backpacking tents. They are both 3 season tents. One is the REI half dome the other is an old Eureka one man tent. The Eureka does not have mesh in the body and it's a small tent which I use in cold (not winter) weather. Because it is small and has no mesh it is a warm tent, too warm for summer. When I crawl out of the tent on a cold morning it is noticibly warmer in the tent than outside. The newer tents are all about mesh for venting. This makes them cold.
I also have 2 sleeeping bags a 15degree down mummy and 35degree down rectangular. Obviously different bags for different temps. I use a BA insulated air core mat and or a Thermorest. As others have mentioned you may need to use 2 mats.
Sleeping bag ratings are only a guideline. You can think of the rating as more of a survival rating not a comfort rating.
With the right combo of sleeping gear you can stay warm in very cold temps.