Rearing when inside the trailer?

It sounds to me like you are doing all the right things. It may take a long time for her to become more comfortable in the trailer. You might try putting a quiet horse on one side of the trailer before you load her. If she sees another horse eating quietly beside her, it may help her to settle down and eat her food without so much worrying. I would continue feeding her in the trailer every day and don't even think about closing her in until she is much more comfortable with the entire process.