Your horse is rearing because you are trying to 'hold' her still or 'hold' her back with both reins. This gives a 'reactive' horse (one that is upset and not thinking or 'responding) no where to go but up -- so she goes up.
Read this entire thread, especially page 3 where I explain exactly how to teach the 'one rein stop'. It has to be me ............I am so discouraged
If you follow all of these steps, you will be able to get this horse under control without a fight and without causing her to rear up.