First off, welcome to the forum
Fiinx, I am willing to bet money that your mare's problem doesn't have anything to do with the bit, problems like this seldom do. First thing you should really do is have her teeth checked by a dentist and check her back to ensure that she's not in pain somewhere.
After that's done, if her teeth/back aren't the problem, then what she needs is some re-training. If it was me, I would put her back into a simple snaffle (smooth, not twisted) and go back to working on the basics. Get her supple and responsive on the flat before you even look at the pattern again and when you do start her back, start her back slow as if she had never been patterned before.
Also, not to sound harsh, but if this is a sudden onset problem since you started riding her, perhaps you need to have a professional help you to re-evaluate the way that you ride just to see if you're doing something to cause this problem.