Our new horse has issues with backing as well. I have found two solutions that may help - hopefully.
1.) Take two leadropes - one that is on his halter and another one to clip on. Have another person help you with this. Have one person hold the leadrope tight and stand out of the way outside and to the side of the trailer. You (or someone else if you would rather be in the trailer and trying to help back him from the front, but if you don't have to then I wouldn't) hold the other leadrope and stand on the other side. Both of you apply pressure on the leadropes and one person (or both) keeps asking softly and steadily "back." Both of you apply pressure to the leads but don't let the horse turn his head - then best you can. Just continue asking to back holding the same amount of pressure for a few seconds. If he still doesn't back then add a bit more pressure wile still asking to back with your voice. As soon as he makes a move to back, release. Then repeat again and wait for a little more movement than his last one, then release again. Keep doing this until he backs out!! :)
2.) As he gets better with #1, then take a long leadrope, swing it around over his neck or withers, and either tie it back to the halter or slide just slide it through the same hole it is attached in. Hold it with one hand and apply pressure back and ask him to back. If it helps - sometimes the horse needs to think you are moving as well - then walk in place. Sometimes if the horse hears your feet moving then he think you are walking back and maybe get out of your way.
🔫 Don't Tread On Me 🐍