OP, I agree she is testing you. Correct her when she is misbehaving. She knows better, so you could try a spank with a crop, but it sounds like she may just react to a spank with more bad behavior, which is what my testy QH does. So the other way to correct is with more work.
More work is lots of turning, like small circles, s curves, figure 8's, etc. Make her work near the area they want to be(barn, gate) and rest away from it.
If she ever does something that makes you feel unsafe, jump off and make her work on the ground (lunge small circles, yield front and hind, backing up). Be intense (not angry)and make her move! I always kept my mares halter and lead rope on and wrapped on the saddle horn so I can jump off and work her at any moment. I was recovering from an accident and just didn't feel confident when she would threaten to buck and rear. I still don't think it's worth the risk of injury, so I hop off whenever I feel unsafe.
The other thing I want to share is that April also had a terrible canter. She kicked out and I thought she was gonna buck. Even when she was honestly trying, she was so rough I was afraid I would fall. I read somewhere that riding up a hill could be helpful. So I found a nice hill in a neighbor's orchard, not too steep, gradual and inviting. The very first day, first time, there was great improvement! We worked that hill every day, starting short and going longer to build her strength. She actually seemed to enjoy the hill as she hated the arena!
April is a great horse now, beautiful canter. She only tests me a little and quickly gets in line. And I feel like I can handle anything without fear because I just get off.
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