I agree with weefoal that the backing is just a habitual evasion of work. Lots of groundwork and establishing a soild "forward" cue and response from the ground. My suggestion as far as getting her going forward under saddle would be the squeeze, cluck, smack method. On a loose (as in sloppy, as long as you feel you can stop her) rein, to be sure that she isn't misinterpreting your contact, gently squeeze your legs. If she doesn't respond, or backs up, cluck to her while you mantain the squeeze. If she doesn't respond to that, or backs up, take your stick and smack her behind your legs with increasing pressure until she gives you the right answer (forward motion, even a fussed hop forward. Reward the smallest change). I would personally use a string with a popper on the end, similar to but floppier than an over/under, it's easier to flip back and forth and reach both sides of the horse. You can tie the string off to the saddle horn or front dee on your saddle, anywhere handy, until you need to use it on the trail. The progression of cues teaches the horse to respond to the first one, since if they don't, there's a storm coming, lol. Eventually, the stick or string is unnecessary. I would start the exercise in a more controlled area than on the trail, an arena or even your mare's pasture, just in case she overreacts in the beginning.
Hope that made sense and helps a bit. Good luck!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown