We get about 6 months of winter here too. To keep warm, I have a pair of tall Ariat winter riding boots that are amazing. They are warmer than my day to day winter boots and incredibly comfortable. Good thermal underclothes also make a huge difference in staying warm. When the weather is moderate and not too windy, I wear fleece lined riding pants or fleece lined jeans. When it is colder, I layer up with a pair of pull on snow pants as well. I have given up on equestrian gloves made for the cold and use light weight snowboarding gloves instead. Even though they are "poofy" they are still very light weight and easy to hold the reins with. I also keep a second pair inside my jacket next to my body when riding so when my hands do get cold, I switch them out. As for a coat, down is excellent, but I use a hip length textile coat I purchased from an outdoor equipment store about 15 years ago. I have an older helmet that is a little large for me that I save for winter riding because I can fit a bella clava under neath it. This way, I can keep my head, ears and face reasonably warm and still protect my noggin.
Dressing for the cold is a challenge, but once you have a decent system worked out it makes winter riding much more enjoyable.