So I've owned 3 horses before, all I started from the ground up and my 2 mares were angels, no spook or bolt from their very first ride. My gelding was sound sensitive and a bit crazy and studdy but I have gotten him over it. Now to help my landlady out I agreed to work some horses for her and I think I'm a bit over my head with this mare. She is a 6 year old TWH and is very forward. Amazing in the arena, she had 30 days ground work on her and 30 riding by a profession trainer about a year ago I think but has since done nothing. I took her out once by herself a few days ago bareback for a short ride and she was a dream, jumped all the sticks and twigs but nothing serious or nothing I couldn't handle. Today I took her out again with a saddle and she was almost unmanageable. She balked and bolted at everything. Spooked down hills to get away from fallen trees. Reared, spun and bolted at rocks. If I talked on her back or broke branches she took off on me. Would NOT go over even ankle high fallen branches. I was riding with her pasture mate who has the same training but takes everything in stride and even when I got my BF to ride off with her there was no getting this mare over things. It would take me anywhere from 5 to 30 minutes to get her through a space between rocks or over sticks. Even on her second time over things she still spooked at them. She was shaking the entire ride, completely tense and just... I tried to keep calm but I was almost scared myself. I plan on riding her once more to see if it was just an off day for her but I think she's going to be a huge project horse and I'm only here for a month. On top of that a 12 year old girl who used to lease my mare wants to buy her. She couldn't handle my 5 year old 17hh mare who never did anything and now they're going to buy this horse because she's pretty. I'm getting frustrated at the situation, I'm pretty sure this girl will get hurt on this horse and I'm not sure what to do.
Should I keep taking her on trails? Work more in the arena? Which by the way she doesn't spook at anything in the arena. Uhhh any advice would be much appreciated!