My old pony used to be like this, he was not worried or scared, just would rather be out😂
If your horse is the same & is just being naughty my tactic is to load them last (when everything else has been packed) and leave immediately after, if they start kicking when traveling drive a little faster and they will have to put their feet on the floor for balance then unload them as soon as you reach where you were going to.
Obviously if your horse is scared or has had bad experiences this is not the way to go but some of them just try it on! After doing this for a while my pony gave up & travels like a saint now- just wanted to see what he could get away with😂
If possible, park the trailer (attached to the jeep) in her field/closed off yard. Take all partitions out and leave both ramps down. Then let her loose and give her her food (both forage and concentrates) in the box where she can see you putting them in. If you can, do it for every feed. If not, as many as possible. Soon she'll start associating the box with food and it will be a positive experience.
When she starts to happily eat her entire feed in the box on her own, you can start purposely putting her in the box to feed her.
When she's happy doing that, you may even start putting in the partition before you feed her.
It may take several weeks but it's rare to see that method fail unless there's a serious cause. In the mean time, have a think about the cause. Was she in an accident? Has she ever lost her balance in the trailer? Ever had a fright? Have you ever had to break hard with her? Do you, or someone in the past drive too fast with her? Basically, have a think and see whether or not the cause can be eliminated. If the cause is ongoing, then no amount of work to make her stand quietly will help.