Originally Posted by mbender
Get her focused on you. When you see she is becoming unfocused, change direction. Do circles, transitions of walk, trot, walk, canter. Do this close to home or even on the property before going out on trails. Have her bend her neck side to side while standing still. My horse trips too when she is distracted but I ride her through it or start to talk to her to let her know I am up on her back and she needs to pay attention to me.
That's pretty much what I would suggest. Right now you have a horse that requires a rider, not a passenger for the trails. Give her a reason to listen to you. It might be a while before you can happily mosey on. My mare is a lot like that as well, so we'll do all the work we might do in an arena, outside of it when we're on the trails..
Go up and over and under and through and around whatever (bushes, trees, stumps, rocks, logs, etc..) you can find as well. She'll have to pay attention then.