When I read the first few lines of your post I had to look and make sure I hadn’t somehow started this thread myself!
First and foremost, yes she will get over it! The more people I talk to, the more I find this phase seems to be very common in 5-6 y/o warmblood mares. My big girl, a Holsteiner, is also 6 and has been through everything from rearing, backing up, kicking out, slamming on breaks to halt from a canter, you name it. All to avoid going forward. Took us a long while to figure her out but she’s now incredibly responsive to my leg. In fact I was recruited to warm up horses for an IEA show at my barn last weekend - fantastic horses, true schoolmasters, and I was shocked that not one of them was nearly as responsive to my leg as my mare is. Which is really something considering it used to take 20 minutes to get her to trot.
Couple tips, some of which have been touched on above:
**Make sure you are not nagging with your leg! At any gait. Put leg on and expect forward, and take leg off the moment she goes faster. That way your leg really means something.
**You describe your canter transitions as: leg, mare trots, crop, mare keeps trotting, more crop, mare kicks out and eventually canters, praise. This is perfect but is only step one. Once she canters, praise, then back to walk. Then re-ask, starting with just light leg. You may have to go through the whole process again, if so, that is fine. But then again come right back to the walk and ask again. What should happen is after a few attempts, she will canter sooner in the progression of aids. Maybe attempt 3 she canters on the first crop, then attempt 4 she canters with only your leg. That’s when you let her walk a little longer and give her a big reward. Without going back and re-asking once she’s cantered the first time, she thinks the aid to canter is “crop crop crop”. If you go right back to it, she will understand that yes she’s a good girl for cantering, but you need her to do it the first time you ask. I hope I explained that in a way that makes sense!