I would start with a good dose of an Ivermectin dewormer. It will kill any blood sucking parasite from an insect on down.
There are little flies that get in some horse's ears that can make them a bloody mess.
I would start with this: Give her an Ivermectic dewormer and take her temperature. If she has any kind of an infection, she will have an elevated temperature. Mature horses generally run little above 99 degrees. If she has a temperature of 101 or higher early in the morning while resting, she needs a Vet ASAP. Even when an ear is not infected (actual ear infections are very rare in a horse and would include drainage), ears with bugs or tick are sore and a horse will not like them handled.
If her temp is normal, work on training yourself and her to let you handle her ears. If you can't, you either need some competent help or a very low, mild dose of tranquilizer. I say a low dose because just 'taking the edge off' usually lets you get the job done and the horse learns in the process.
If you are still having difficulties getting her to let you handle her ears, you may need to get a trainer to help you get past this hurdle. We recommend 'approach and retreat' as the place to start.
When I have horses that are bad to get bugs in their ears (or ticks), I put a little Ivermectin dewormer on my thumb (or a pour-on used in cattle) and smear it on the inside of a horse's ears.