Problems getting on the bit, lacking self-carriage
I have a young quarter horse mare who often requires a rather heavy hand to get her on the bit. I know she's not accepting easily because not only isn't she foaming but I can feel her pulling. She often pulls to the left or right, or down, or hollows and throws her head up. Due to some minor arthritis (which is completely under control) she is stiff especially during the first few minutes but even when she warms up, she can still be tough. When I do get her on the bit consistently at the trot, I soften my hands a bit but still keep the pressure (firm outside rein, sponge-squeeze the inside rein) but she often loses the contact by evading the bit. It's a horrid cycle. Last week I rode for 5 days and my arms felt like they were going to fall off by the end of the week. I haven't had a lesson recently due to my trainer being unavailable but when I had the lesson, my trainer told me not to tense my arms but without making them firm (as if I were lifting a 10 lb weight), I couldn't get her on the bit. I'm also concerned that this heavy hand isn't getting her to carry herself properly, but that she's relying on my hand. She's also minimally responsive to the aids, particularly in the downward transition from trot to walk. She was jigging so much (perhaps a stiffness problem) that even a firm pull back on the reins had an extremely delayed response.
**We started a joint supplement on Sunday, so hopefully the glucosamine will help her out a bit and then when this horrible weather passes, I can see where she stands a bit more accurately... I'm hoping to separate the physiological soreness from the behavioral issues.
Even with the lack of impulsion, her walk on the bit doesn't require as heavy a hand, except for the initial asking her to get it.
How can I solve this problem? It's been going on for much too long, and it creates a very frustrating and unpleasant environment in the saddle.