My horse won't bend to the left?

I have a 13 year old appendix mare and I always have a really hard time getting her to bend to the left. she will only do it if I put an enormous amount of pressure on my inside leg and really pull on the inside rein, but when i do this she'll come off the rail and I have to use my outside rein to get her back on the rail. I asked my trainer about this and she told me I also had to keep contact with my outside rein to keep her from coming of the rail. I did this and its been helping, but the real problem is her just not bending to the left and It's exsahsting having to constantly push with my inside leg all the time just to get a that left bend, espescially at the canter where its the worst please help me :(