In my opinion I wouldn't use any bit such as a mikmar on a five yera old. Gag big and draw reins! Hmm I think you should just use a simple snaffle and work on control rather than relying on artificial aids to slow your horse down or get her under control. Just try some lightning exercises to get her to full control with a snaffle. There's nothing more respectable than someone who can control there horse with the less amount of tack.