First, can you remove the divider in way the trailer would look wide and inviting? That helps lots.
Since you don't have bond/trust/etc (new horse) you can try to use grain to load. Not the best way in world, but you have to haul it anyway. When I first hauled mine we put rope besides her so she couldn't back up. But(!) she was very young (a yearling) and I already owned her for month (and messed with her all the time), so I'm not sure it's good way for your horse.
Here is an article from guy who does loading clinics. I've seen him once putting abused horse in 2-horse straight (it took 4 hours and 4 men to lift that horse in trailer to deliver to the clinic). That was absolutely amazing. Took him 40 mins, but horse did it herself and was no drama at all going on. http://www.feedscoop.com/kh2.htm