In that case, I would have your dad be doing most of the work right now with you helping. Get a solid base on the ground so she learns to trust and isn't worrying about the monster that is going to jump out of the hedges and eat her.
Once she is solid on the ground, start her in a round pen under saddle. Let her feel the saddle without a rider and make sense of it all. Once she is solid here, your dad can begin to ride her and let her figure that out.
From what you said, it sounds like an inexperienced horse that doesn't know what's going on. If somethings not sticking with her, don't move onto the next phase. Some horses you learn faster than other but it sounds like this girl may need a bit more time before you move ahead.