Had the exact same thing with my cutting trained barrel horse. This is going to sound wierd but it worked so well.  We stood on the ground with a halter and asked the horse to go around us.  With the nose tucked towards you we held on to the tail and when the horse wanted to spin you just "shooshed" them on with the tail.  Basically it looks like a piece of pie with you having your arms out forming the triangle and the horse is bending like the crust of the pie.  We did this at a walk and My horse was confused the first couple of minutes but when we kept him walking and had his nose tucked and had hold of the tail, forming the arc in the horses body for him, he would start walking a perfect arc'd circle. 

It's a learning thing. They just have not been taught how to arc a circle. It had all been roll back and spin stuff.  Try it and let me know if it works for you.  It sure worked for us.  My husband did this every day for several days and then I slowly took it to me in the saddle.  I still have to work him on that because he will roll back if I don't remind him.

I had horse problems similar to that before. I hand my horse over to my husband & say "Fix this, please." That probably doesn't help you out much. tee hee! 

Jared said that he would "bit her up" back to the horn for about 15 mins right before you ride for a few days to a week.

After that few days, "bit her up" back to the cinch ring one side at a time for a few days, each time before you ride. Just give her room to give more on her own, not too tight.

He said that should teach her how to give to a bit, since no one has probably shown her before you got her.

That's what the hubby said.  Have fun playing with your baby!