You might want to try taking your valve cover off and see if you have a broken head bolt... if not, it's your gasket. Get a pressure check done, and that should do it. An additive isn't the answer... additives usually only fill cracks in aluminum rads... not in gaskets, but you might be lucky. There are a few coolant "stop-leak" products available, but when you're driving in a Colorado snow storm, it'll be in the back of your mind that you have a blown head gasket being held together with "stop leak".