So my pony never bites me or anything. But she always seems to be waiting for the moment that my guard is down. This is only when saddling, mind you. She's quite girthy, always has been. But since I've had her, I haven't let her get away with touching me at all, so she doesn't. But whenever I tighten the girth, her ears go back a little, then she turns her face to bite me, but as soon as I look at her, she looks away. If she gets too close, I smack her away, and she moves her head away for a moment before trying again. She never bites me, but she always thinks about it. And I can't punish her, because if I smack her, it doesn't phase her and she just turns away then back again.
Is this a behavior I should treat the way I have been?
How should I properly handle this?