Is this mare turned out with the other horses? Is she dominant or subordinant in the herd? Is she defensive of herself in any other way, such as does she pin her ears when you approach, or put the saddle on or try to lead her? Do you get the feeling she is trying to "do unto you before she you do unto her?"
I mean, is her kicking a self defense mechanism or do your really think she wants to attack you? If so, wouldn't she have charged you and wouldn't some of her kicks hit home?
I think (and this is not my idea but is based on reading I have done. So that means it is just theory as far as I know from personal experience and take it for that only) . .. That she feels very much pressured and needing to defend herself , pretty much all the time. This is probably as a result of the overly high pressure techniques used by that trainer.
She is now in self defense mode, because she thinks she has no other options.
What Imight do is take her into the round pen and do . . . Nothing.
Don't press her at all. Let her look all around the pen, looking for help, just stay there and hope that she may turn and see you as the calmest, nicest place to be. If you need to , move her away from the gate, and if she kicks do nothing (except stay clear) don't increase the pressure on her at all. Ask her to move a little, if she takes off kicking and bucking . . Do nothing.
If she zones out and starts to run around and around, let her do it for 5 or 6 trips, then just interupt her and change the direction by stepping into her path.
You just keep turning around , stay facing her as she circles, but keep you body in a nuetral frame. If she cocks an ear on you or looks at you like she would likd to come in, take one small step away, back, and see if she draws in.
Relieve the pressure the tiniest amount any time she looks at you , BUT, if she turns suddenly and charges in at you too fast., put your hands up firmly and stop her. She needs to be invited in and needs to come in peacefully.
If you get her to stop bucking and look at you but won't come, you go up to her, pet her then walk away. She may hook on. If she does, take a short walk around the pen, somemore petting and quit!
Just see if the feel isn't different if you go into the round pen with her with your idea to be the calmest, steadiest place there is, not to add to her pressure. If she kicks and bucks and you do nothining and she does nothing but kick/buck and makes no change in attitude, then nothing is lost by at least trying this.