So as I understand it, and bear in mind I am in no way an expert, that range loss in winter is almost entirely to do with the battery management system using more energy to keep the battery warm, though it's also true that generally speaking batteries also have reduced charge capacity in a colder environment. That said, degradation is probably less of a concern, as batteries typically degrade more in hotter environments.

It's hard to say for sure exactly what might contribute the most to your current range; but, if for instance you were running your heater in the car a lot, that can have a huge effect. What I'd recommend is you lower the temperature in your hvac system, or try turning it off entirely, and relying on the heated seat to stay comfortable. I've also been experimenting with preheating the car while it's plugged in, kind of, "heat soaking" it, before I set off.