Hi there. Late to the forum and you might have had success by now, but I can tell you what we did with my gelding as pushing/pulling, cajoling, begging and tempting with food had no impact. We made a ‘runway’ for my horse into the trailer with light colored lead ropes (one on the left and one on the right). Sounds strange, but for some reason he really did not want to step ‘outside the lines’ and made his own decision to go forward. Loads fine into anything now, but for a few months, that was the norm.
Best of luck,