You just gotta bite the bullet and go out there... Put an extra moveable layer on, and head out
Have a goal in mind, before you go out; that always helps me...like say, I will go out, and have continuing teaching everyone to bow, and lay down...and with three to work with, that gives me an easy 30 minutes of training (as those sessions are short for each horse). If it's chilly, that is a perfect amount of time, and I have accomplished something, other than going out and just grooming, and lunging (which I will do as well), and takes up another 40 minutes. Other times I will just go out with doing gait changes and stopping in mind (for my mare), so with warm up, that will give me about 25 minutes worth of teaching time. Add in lunging, prior to riding, and groom and cool down time, that gives me no less than an hour with my horse; and in cool weather, that is way more than plenty! Course, where I'm at, I haven't experienced lower temps (daytime) than 25 degrees. BUT I grew up in MN, so winter time was BRUTAL! But I still spent alot of time riding; when you grow up with cold temps, you learn just to 'brave' them so you can ride!