My horse had the same problem. We went out on the same trail every day for three months (literally every day) while I was rehabbing a joint issue. He would get to the gate where the trail started and it was like he hit a wall. He wouldn't go forwards or sideways, just backwards. What I did was turn him around so he could see the barn and made him back up the whole way. If he wants to back up, then he can back up my way. Then we did lots of bending and leg yields to make him understand that it's a lot more work to refuse. He still does this somedays, but I at least have a solution.
I have to ride in spurs and a bat and somedays I feel like a horrible person for getting after him, but it has to happen. I think he associates the trail with work and he doesn't want to work. When I take him off the property to go cross country schooling he is totally fine to go where ever I want him to go because it's fun. Maybe try to go to a different place once in a while? I haven't read all of the posts so I don't know if someone already said this