Perhaps a first look at why condensation occurs. When water vapor encounters a cool surface, the vapor can turn into droplets, which coalesce onto the cool surface. Its a function of the dew point
Let us assume that you have properly acclimated your sextant to the ambient temperature. That is, you let it warm up / cool down to the outside temperature. That's standard advice, so please do so.
Your sextant telescope, held to the eye, leaves the horizon mirror directly in front of your presumably warmer breath. Moist too. So as you blow warm, moist air from your lungs directly onto the cool mirror, condensation can occur, when at the dew point.
Anti-fog coatings can help, but so will plenty of airflow. Keeping your mirror crisply clean will help too. Holding a sextant endlessly to your eye can be tiring. Not holding it there will reduce the condensation as there will be plenty of ambient airflow and less human breath on the mirror. For lunars, take Frank's advice. Look thru the scope, take the sextant down from the eye, adjust the sextant, and then back up to the eye. Use both hands to hold the sextant up. Short bursts of observation with intermingled with rest / adjustment is far less fatiguing and, in your situation, will reduce condensation.
Of course, you could always just stop or minimize breathing during the observation. Please remember to resume breathing at the appropriate time! :)