Do you believe she had any under-saddle training before you got her? If she didn't (or if she was never loped under saddle), then she's likely just really unbalanced.
I can only compare her to a single horse I've rode since she was the only one that age I've started. That mare was about 10, maybe a little older, and had never been ridden. Her walk and trot were great, but her lope was torture for both of us. Choppy, unbalanced, rushed, hollow, etc.
The thing that I found that helped her more than anything was just loping a lot. I took her out in the country and loped her until either she or I couldn't anymore. I'd let her walk until she got her breath back (or I did) and then we'd lope again. It took some time, but she finally figured out how to slow down, level out, and relax at the gait. She never had a good canter, I'm unsure whether it's due to her age or her conformation, but what I ended up with was exponentially better than where we started.