I have the same problem with my paint horse. She is 7 and has always been the "baby", and she is not confident with leading on the trail. My problem is that our other horse, the arab, is the leader at home (she's the old one), but she will not lead on the trail, no way! So the paint has to lead, and she's not happy about it. She will start turning around, backing and refuse to go. Then once we are away from home and on the trail, she's fine.
I think that the packing order has a lot to say, there are followers and leaders. My paint didn't have any problem going out when we had the other arab, she was the leader at home too but was fine being the leader on the trail as well.