Hi trailrider from CO
I used to live in CO and did lots of trail riding there. So many gorgeous places to ride!!! What area do you live in? I used to live in Castle Rock.
Barn sour is a terrible thing to get them over at that age. Sounds like she has gotten away with it a long long time. One thing that helps, take her for lots of little walks, you leading her. Find a spot and let her graze, just relax. Let her look forward to trips with you. Then ride a bit, get off and let her graze a while. Most horses learn to love these little jaunts and learn to depend more on you.
(then later if you really have to battle her, she will take it better)
Hopefully you can find a riding buddy to go out with you until she learns that you aren't one of those riders that just lets her go back to the barn when SHE decides. You and your riding buddy can go short ways out and YOU decide when to turn around.