I am not all that experienced with trailer loading, but I think what your are doing broken into more of the "approach and retreat" method will have the horse feeling less forced in. I mean you get him to take a step forward, then you ask him to step back, you do this quite a few times, progressing to having him go further in, but asking him to come out before he decided to come out on his own.
There's a lady called Ellen Ofstad that has several videos on Youtube (about 5 minutes each) that show her approach to trailer loading.
You know,, I cheat. I put treats inside and my horse goes in. Hates it, but goes in. I am a lazy cheater!