I feel for you. Years ago I had a mare that would load in anything that you asked her to get into (even an old single wide). Then when I moved from TX to AZ, we had trouble. We stopped in NM for the night and she had been in the trailer for 10 hours. When we went to get her the next morning she wouldn't let me catch her (she used to walk up and put her head in the halter). Once caught she wouldn't load past her front feet. Very frustrating we tried for 5 hours when some ladies with a large slant load that had been staying there offered to help. We got her in once but they didn't close the door and she backed out just as fast and would not go back in. LOng story short she ended up riding in a strangers trailer (Since she loaded right into her 4 horse slant gooseneck, but not into our 2 horse slant bumper pull) from NM to northern AZ (I live in central). The ladies were on there way to a clinic put on by Lee Smith (I believe that is her name) She uses the Natural type training. Anyways I went up to the ladies place a day after I got home with our 2 horse straight load and she used her for a clinic demostration. Within 30 minutes she was loading in the trailer and I was crying (I know silly). So my best recommendation is to find a video or speak with a trainer and just start working with the pony and the key is not to get mad or frustrated and act like you have a Million years to get the pony in the trailer. I did find that it was just as much me that was the problem and it was her. All the natural trainers do it about the same way and it is a good place to start. Good luck and remeber dont try training the pony to get in the trailer when you have to go some where. Now if she sees the trailer open she will walk in even with out a halter. The key thing is do not use feed or treats to brib the horse in.

Good luck.