Try joining a gym for the winter like you said. The cold can do weird things to you sometimes. Coming to boxing with a mountaineering background, I can tell you I've pushed myself to the absolute limit in cold weather, snow, ice, all sorts of stuff.... Little more extreme than training in the garage in the winter I guess lol.... Your body is working extra hard to keep your core temperature up, and so it would make sense why you feel more sluggish in colder temps..... anyway, nuff ramblin', try the gym thing for a bit. Might even find its got other things to offer than just a more comfortable temperature to train in.