The approach and retreat is a great idea. Just remember to back her away before she gets scared. Notice the small signs of stress- pinched nose, hard eyes. When the head comes up high, you are starting to over expose her and back her away calmly.

The important mind set for YOU is that you have no where to go, no set time to get her loaded in. If you don't get her loaded that day (as was already said) as long as she is relaxed and trying in baby steps for you then quit and start again the next day. This is about building trust time with you and your horse. When you do get her on, you need to make sure you take her off again. Keep everything calm and relaxed. If she is at any point not calm, then you need to reassess your emotions and breathe. It's not about the trailer, it's about trust and having a relaxed mind.

"Keep an open mind so that your thinking can be flexible, but not much that your brains fall out."- self