It will take a bit of time but can be fixed if you have the patience.
1.) Get a wireless clipper.
2.) With her on a lead line feed her treats while you turn the wireless clippers on. Once she's OK with the sound (make take an hour may take days - just try not to make training session longer than an hour) you're ready for the next step. Another tip - you don't want her to feel confined so do NOT tie her - have her in the stall on a lead line, allowing her to walk around you when she gets nervous.
3.) When she finally relaxes (to sound) press the clipper against her neck with your hand in between. This way she feels some vibration but not all of it. More treats and quiet "Good girl".
4.) Once she's OK with that clip legs. If that doesn't bother her clip muzzle while feeding her treats and talking quietly to her.
5.) Once muzzle is OK try working under her jaw and/or around her eyes.
6.) Before trying to clip her ears you'll need to get her to accept you hand lightly passing over her ears. Again - use treats and only press her until you see the first sign of it being an issue. Don't go past that point (of it being an issue) until she's OK with it. Since she's had very bad experiences this will probably takes a few weeks (or more). Once she lets you touch her ears on outside you might want to rub your fingers into her ears on he inside. They generally love this since it "scratches their itches" (Ears get dirty inside) but she'll be reluctant, so go slowly while talking very quietly to her (no clippers).
Don't try the ears until she's perfect with rest of her head being clipped and really good with your touching and rubbing her ears. Once she's ready for ears to be clipped - squeeze them together (dulls the sound plus prevents hairs from falling into ears - which also itches/tickles). Just clip the hair sticking out of the ears when they're pressed together. When she's good woth that you can start trimming the outline of her ears then gradually go for the hair inside the ear.
It'll probably takes a few months but will be totally worth the result - just pay VERY close attention to mare's mood - worried is OK, scared is not.