Before a horse can go on the bit, she needs to have the foundation to adopt the carriage correctly. Start at the base of the training scale, at relaxation. If she isn't relaxed, she can't do much more than jog around with a hollow back. You said that you are selling her since you are too big for her, so I might point out that if she is carrying too much saddle/rider weight for her body type and size, that could be affecting her relaxation, rhythm, and balance, making it harder for her to carry herself well. It's like you trying to walk with the posture of a ballerina while carrying a week's worth of camping gear into the woods - it just doesn't work well.
Another thing to be sure that you are doing is not being too assertive with your hands. No pulling, just correct hand position, and a soft "handshake" contact with the horse's mouth. Contact isn't all about hands and reins, though. The horse should be accepting and responsive to your legs and seat as well. I personally like "on the aids" better as a description of the correct posture rather than "on the bit." With that conversational contact, ride her forward, creating impulsion with your seat and legs.
Here's a good article on getting the horse on the aids. The Art of Classical Riding--On the Bit
If you click on "The Riding School," in the upper left hand of the blue bar at the top of the page, there are oodles of articles on more foundational work, if you find that to be lacking.
Also, consistency takes time and patience. The horse is learning a new posture, perhaps radically different than what she's used to, whether she learned it before or not. She may take some time to build the right muscles back up.
I hope that was helpful to you, and good luck!