If she's not balanced you won't get a clean transition.
Do more work on your walk-trot-walk-trot-halt-trot transitions, get some leg yield and a little shoulder in happening. Get her in self carriage, off the forehand and willing to take some increased weight through her hind quarters.
Establish a half halt in trot - this is not 'pull back then kick forward'. Steady with your core, when you feel her steady, ask her to go forward again into the bridle. Keep your hands dead still and keep an even contact to the bit.
Multiple changes of rein on figure of 8's, 3 loop, 5 loop and 6 loop serpentines. Introduce smaller figures such as 15m circles and then 10m circles as she becomes more balanced.
Its all good and well to ask for the stretch down from the neck, but long and low work is actually much more complicated than just stretching down. 80% or riders will get the stretch and the horse will fall straight on the forehand. You MUST be able to hold the power with your core, keep the hind quarters engaged and ride the horse into the bridle while getting the stretch or it is a useless exercise and simply putting the horse even more on the forehand.
Before worrying about canter, start bringing her up a little. The frame can stay quite long, but she MUST be off the forehand and balanced.
When asking for a canter transition on a green horse, I'll do it from leg yield or coming out of a 10m circle when the horse HAS to be over the hind legs and balanced. Sit quietly in the saddle, simply lift the inside hip and allow the horse to roll into canter. What tends to happen to riders on unbalanced horses is once the horse takes even a single faster or unbalanced step, the rider tenses, hangs on, grips with their knees... which turns into a spiral of gripping and unbalance and tension. The horse perfectly mirrors the rider's error.
Try to think of sitting back on a Harley Davidson motor bike when you are picking up canter, just roll your hip into it and allow the horse to come up underneath you.