Don't put a time limit on getting your horse in. Every step closer give them a pat and reassurance if they step backwards make them walk backwards further so they learn walking in is much nicer. Once they are in give them their feed in there and make it as nice and relaxed as possible. Do it as often as you can (when ever you have lots of time). My favourite saying is make the right thing easy and the wrong thing hard 😊 Good luck!
Don't force your horse. If you want this to be long lasting you have to make the horse " want " to go in that trailer. Do some horsemanship to make your horse trust you and build yoy it's confidence before it's time to go in the trailer. And after awhile when you start with the trailer don't go all the way in. When your horse takes a step in the trailer, reward. Back out and restart. That have worked for me but I'm not saying it will work on all horses but I hope you find a way.
Before trucking my young boy, for a half an hour every few days we got him to walk through barrels and poles that are placed to seem like a box just more open. Then when he called down we would put the poles closer together. Eventually he walked in and even backed out perfectly so we moved on to practicing in the horsebox. Hope this helps
My old pony used to refuse being loaded onto the trailer. She would stop, back or throw herself around.. We simply just put one lunging rope at each side of the trailer, which worked surprisingly well! She went straight on :)
Not saying this will work for all horses, but it's definitely worth a try!
My horse was a complete brat to load alone and with company but given more trips long and short with a calm companion horse he's improved immensely. When i have to take him to a show alone and he won't load i would load a horse with him and then back out the other horse while distracting my boy with a lick or something. Same at the show on the way home but he's normally not as bad on the way home. I think it's all about confidence.
Horses do best what they do most