If you’ve ruled out any physical problems (have you checked her teeth/bit?), I’d work on lots of transitions and getting her to relax. Take lots of walk breaks, lots of change of direction/bend, maybe even work her over ground poles, etc. Even on the lunge line to. Anything that can get her thinking about what you want. I have a 23 yr old arab mare that frets a lot. I always have to remind myself to have soft hands and a supportive leg while half-halting and that usually helps.