I've recently started lightly riding my three year old cob mare. Up to now she's been really good and very responsive and relaxed, didn't mind me on her back at all and was really happy walking and gently trotting. I've always known she's had a stubborn streak in her, but suddenly she's started being really naughty - after weeks of nicely accepting the bit when I put her bridle on, she's started clamping her mouth shut and dragging me (and others) around and even throwing her head back like she is going to rear, whenever the bit goes near her mouth. We finally got her to take the bit by taking it off one side and putting the bridle on, then gently slipping it in 'sideways' whilst distracting her with food. Once she's got the bridle on and the bit in, she isn't bothered in the least, also she's not bothered when I put any sort of pressure on the reins whilst riding her. I am concerned she may be a little sore in the mouth after not having been used to a bit, but I am only riding her once a week and the bit is only a snaffle, also there are no sores on her mouth. Do I need to get her a rubber bit, or a 'sweet iron' bit? Or would simply rubbing a bit of treacle or something tasty on there help?