She's not balanced enough yet with you up there. She's only 3 and built quite downhill. Her conformation is going to make it hard for her to do anything that she's not 100% balanced and ready for. All horses have a preferred lead and their other one can be a struggle.
I would go back and do more trot work. Work her on patterns and over poles at a good working trot. Drive her forward so she's "marching" and you feel her back come up under your seat. When turning or circling to the right, lift up your right rein and sit a bit more to the left, to help her balance and not drop her inside shoulder.
Working on the trail over varied terrain will also greatly help her balance and strength. Go through ditches, over logs, and up hills, all at a walk and trot.
Plus just give her time, she's still a baby! 3yrs old is quite young, no matter how mature she looks physcially. She still has 2 more years of growing and filling out to do, even if she doesn't get any taller. Her mind is also young, so don't drill her too hard or too long. Keep lessons short and routines varied, so she doesn't become bored or sour to ring work. Break it up with trails rides or even just little jaunts in the field.
When you do go back to canter work, make sure your position is helping her as much as possible. LOOK UP, shoulders back and relaxed, back soft, legs well under you, hands a tad high with arms relaxed. Start on a big circle at the trot. Get a nice marching trot. Sink your weight into your outside seat bone or step a little into your outside stirrup. Let your inside leg relax and your inside (right) hip drift forward a tad. Lift up your inside rein (not back, just up). Cue with your outside (left) leg when you feel her outside hind leg reach and step forward (her outside hip will drop). That is the best time to cue to get the proper lead.
If she picks up the wrong lead or cross-canters, bring her right back down to a trot, let her relax into the trot, then ask again. Once she does pick up the correct lead, praise her heavily, lots of rubs and pats on the neck, and quit for the day to let her "think" about the lesson.