I lead the horse on, and when we get up front where the tie ring is, I pull the partition closed BUT NOT LATCHED behind me, so the horse thinks its closed. Tie the horse. Slip out from behind partition. Latch partition closed.
I load all mine as well... this way ^^^^^
If they are babies and just getting used to trailering tied, as my babies trailer loose with their mother tied.. I first start trailering them loose alone, then once I know they understand how to tie, tie them and leave them the entire trailer to move around but learn that they are tied.. and eventually teach them to understand the divider (i dont introduce the divider immediately just incase they do freak and break loose and want to try to climb out or turn around (dont ask.. ive had a horse try). They all understand it.
This is how I load in my 3 hrs slant, granted I have an escape door at the first stall so it may not be an option for the OP. I tie a big cotton lunge line around the divider that goes to the last stall. I walk my horse in to the first stall, when horse in position. I pull the lunge line that pulls the second divider closed. I slip out of the escape door, then turn around and clip the lead line that is always there for trailering(also in a Blocker Tie Ring) on to the halter, shut the escape door then go around slip between the two dividers and kindly ask my horse to step over by gently pushing the first divider closed.
I had to develop this method because my mare had a loading issue and would unload herself if I left her, or someone else tried to step on to the trailer to shut the divider. I did not want her panicking and pulling back in the trailer so I figured out a way to make everybody happy. It makes trailering by myself a lot easier.