Missed something in my ponies training

Question
Hi i need help, i have missed something in my ponies training but i cant figure out what it is and what i can do about it.  Hes a 9 year old gelding who really dislikes the trailer.  I have worked hard with him and he now follows me into the trailer quite happily, he will stand in there with me making a fuss of him, we have even fed him his dinner in there(so he has stood in there for quite some time, its taken me weeks to get to this point, but the moment there is any sign that the back bar is going to go on he backs off really quickly, he will follow me back on too the point of just being on but not far enough on to be tied, he is really terried of the feeling of being trapped in the trailer, any advice on how i can help him ease this fear would be gratefully received

Answer
I don't know what was done before or what you did, I only know what you say you did and what you think you did.  What ever you are doing is not working, so you need to do something different.

If the trailer has a door then close the door with NO butt bar, get the horse used to that, put it in, close the door and then let it out, after the horse gets good at that then you worry about the bar.