Thanks everyone for the great advice so far! I'm really glad I started reading this forum. I've learned a lot.

My next plan of action will be to have the trailer double-checked when we take it in for spring service and to have my husband drive around with the bucket of water to see if we need to improve our driving. We practiced a lot before we hauled her the first time, but we are relatively new at trailering and always have much to learn.

Once the snow and ice melt (yes, we still have both...), I'm going to work on the desensitization method. I'm also going to hand-walk her a lot to improve her whoa and go on the ground and to get us both back in condition for riding. That should help with trust and fear issues on both sides of the lead rope.

Next, I've talked to the barn owner, and she has agreed to let us keep our trailer attached to our truck in the barn driveway for a few weeks. We're going to go out every night and grain her near or in the trailer. Hopefully this will make the trailer a happy place--for everyone.

I know a lot of this is probably me. I'm an OK rider--would be better if I didn't have some fear issues--and I think sometimes those complicated issues show up when I'm working with my mare. She can be unpredictable at times, and I'm probably overhorsed (to reference another thread here) in the sense that she's sensitive and so am I, so we're both spoiling for something to freak out about; but she's my horse, and I have a lot of love, time and money invested in her. I think we can ride through this. We're also going to have a trainer come to the barn once the weather gets better, and she works specifically with issues of confidence in horse and rider. She comes highly, highly recommended by two different people, and I enjoyed the one lesson I've had with her. Fingers crossed that this will resolve easily.