I get rather good automatic erections, but when I need one for sex it isn't all that hard. When I am with a woman it sometimes takes 45 minutes before getting an erection. While many people find that they do slow down a bit during the winter months, the impact on sexual desire is usually not as dramatic as you describe. But to be safe, I suggest you see your doctor first and get a complete workup to rule out any physical causes. Declines in desire can be related to a number of factors.
Loss of libido is one of the most common symptoms of depression.
Perhaps you are experiencing a condition known as seasonal affective disorder. Due to the decreased exposure to sunlight, many people feel lethargic and even depressed during winter months. Luckily, there are a number of excellent treatments for seasonal affective disorder, including light therapy. You don't say whether you're now in a relationship or not, but relationship troubles often manifest first in the sexual realm. If you are angry, resentful, anxious, or not satisfied in any way about your relationship or partner, a drop in desire would be a very common consequence.