I have had a problem like this before first thing NO WHIPS because it will teach him that loading is a bad experience. Good steps that I took that worked are:
1)Have hay/hay bag at the end to give your horse reward.
2)Even 1 step gets praise.
3)If they back up, back them up further.
4)Even if loaded once do it a few more times so she gets it.
Have a horse who’s confident going in trailers go in first and have the bad one watch. Then lead her in and let them stand there and have good trailer experiences.