best dressage bit for 'heavy' horse

[digitalangel]

im bringing my mare back into work, shes normally hacked out/schooled in a waterford. As she used to drive, she is very heavy in the hand, and is rather like a boat to steer! I think some of this can be put down to coming back into work and finding it hard, so she can become rather unbalanced and 'block' but she is certainly improving.

I want to take her out to a show in a couple of weeks, do to a W&T dressage - i need to change the bit as i assume waterfords are illegal in dressage, so im looking for some recommendations for an alternative? She goes quite well in the waterford, but can still lean on it a bit. Would just like something that will give us brakes if required and help lighten her up in the mouth a bit.

( not that i think she would p*ss off, but hey you never know! )