Help with my barrel racing mare?

I have a mare that's well in her teens and I hope to pursue barrel racing with her. Though she is very hollow and doesn't really lift and bend. I've tried a wonder bit but she shakes her head throughout the pattern. I've used a combination hackamore but she still shakes her head. Another thing she does is when she trots she holds her head high and "bobs" and when she runs she's flat out. Right now I just have her a full cheek snaffle. I can't afford a mikmar bit but is there any other bit I can use that can promote control and bend? She works well off of nose so I was thinking a reinsman ring combination rope nose hackamore. I just want advice before investing in an expensive bit