She already knows what a crop is, that it's meant to make her go forward.
Spend more time on the ground desensitizing her to it. Rub her all over the place and also get her used to it moving around, waving through the air etc. When that is good, make sure that she is respectful but not fearful of it as a tool. Ask her to move her hindquarters around then go back to just rubbing her.
When she's good on the ground go ahead and mount. The groundwork should of taken the fear and or over reaction out, however go ahead and keep her head flexed when you mount. From there, repeat the desensitizing. Rub her all over with her head flexed. If she wants to move that's fine but it will be moving her hindquarters away which is a safer option for the both of you. Keep rubbing till she stands still. Repeat until you can rub and move the whip without her wanting to move in the first place. From there switch sides and repeat. When you switch sides, switch the side her head is flex on. Only when she is good with the whip moving and rubbing her at a standstill should you progress to the walk.
At the walk rub her again, if she goes to react, bend her to a stop and rub till she's relaxed. Then walk off and repeat. It could take a few rides for her to think of the whip as a benign object so just stay patient.
Once she is calm with the whip at the walk, trot and lope can you progress to actually using it. Again start at the standstill with her head flexed and just move her hindquarters around with it, if she reacts instead of just responds, just tap the air till she moves around calmly about it. At this point you need to make sure your desensitization is still working, make sure you can passively rub your horse without a reaction.
From there go ahead and use the whip as you would want at the walk then the trot then lope once that's good.