Cancelaura

So right now I have a macro:
/cast deterrence
/cancelaura deterrence
and it's bound to c, so when I press c it casts deterrence and if I press it again, it cancels it.

All good and dandy, but recently I've been doing PvP and sometimes I will hammer the button when I'm in a stun to use it straight away when I come out, but because I'm hammering it, it will cancel it sometimes > wasted.

So I decided to make a modifier macro to use c to cast deterrence and ctrl c to cancel the aura, but how do I do this?