Of course its winter now but maybe this will help for next year. My horse will only wear a fly mask without ears otherwise she will get it off. She also has a pink nose. I use coppertone Kids Free SPF 80. its waterproof . the first thing she does is try to wash it off in the water trough.
Can’t whoever is putting your horse out put the sunscreen on? We have a cooperative barn and share chores. whoever is turning out that day puts on the fly mask and the sunscreen or the blankets or the boots etc, etc.