My mare is a bad one to float. She also(used to) walk on half way then back off and be too nervous to go back near it.
A few tips I can give
-When attempting to load, don't look your horse in the eye, this is a preditor thing and it can make the hrose feel threatened
-Never get angry, stay calm and constantly talking to your horse
-Reward for EVERY single step forwards.. even if they just pick up a hoof and put it back down in the same spot, ease all preassure and stroke on the neck(avoiding patting,, horses prefer patting and it relaxes them)
-Use pressure release method. Tap on the back of the should with a whip, by tap i mean no harder than say tapping your pencil on a table when your bored. AS SOON as they make ANY form of move forwards, stop tapping and stroke. At the same time as patting, ease on a small amount of pressure to the back of the halter, if they fight and run backwards, just let them go, theres no use fighting,they will win. Just take a step back and start again.
-Your horse will find happy place. For what you guys have said, has been halfway until they stop and run backwards... find they're happy place and see if they will stand there. If your horse runs backwards and wont stand still, dont ask them to walk up so far, just 2 hooves on the ramp is a start. once they find a happy place, ask them backwards. do not let them move forwards, after doing this enough, slowly ask them another step or two forwards, see if you can move their happy place up the trailer, all the time, asking them to back off. This is a kind of reverse phycology, backing up is harder for a horse, and by telling your horse to go backwards, it'll just get bored of it, and want to go all the way on, and learn that it's not a dangerous place, your asking them to leave, then letting them back on.
Gradually build this to finding a happy place, and only asking them to back off one or two steps. stop for a few minutes, then move back forwards, so they dont leave the float each time. Vary the amount of time between each time they stand, and back off, so they don't anticipate it, and back off before you ask