Yep, unloading can be the hardest part of trailer training for a horse, particularly if there's no ramp.
I would first teach her to yield really well & reliably in all sorts of ways on the ground without the trailer to begin with. Then I'd get her going over, around, through other obstacles that she hasn't learned to stress about.
Do it in a non-confrontational way, using 'approach & retreat' tactics rather than trying to force her into stuff she's not yet ready for. The more she 'practices' stressing about something, such as backing out of the trailer, the stronger that association/attitude/behaviour becomes. So it's important to do it in easy enough steps to avoid, or at least minimise her stress & allow her to practice getting it Right.
I'd get her to the point of easily being able to yield individual steps forward & backwards over/off some different obstacles before I'd start again with the trailer. Considering the above, about practicing stressing, I'd be inclined to go ultra slow & ensure she was truly confident with each step before asking for a little more. I'd also do it in very short sessions with stress-free breaks in between, both to negatively reinforce whatever she gives you and to offload any stress so that you don't get that 'final straw' syndrome from low but unending stress.
I'd also consider putting a ramp up to the tailgate, to make it easier for her to begin with. Then baby steps again, I'd ask her to walk up to the float and put one foot on the tailgate/ramp only. Repeat the process over & again until she's confident & relaxed about putting one or both front feet on there, before asking for a little more....