I agree with the others on starting at the walk. Getting her to bend consistantly in both directions, big circles, little circles, serpentines, bringing her nose to your foot (not when moving, lol). And if she seems fine in the round pen, work in the field more. Then do the same at the trot, and add lots of transitions, IF
I remember correctly, a half halt is achieved by sitting deep in the saddle and squeezing your thighs with a slight contact on the reins, you don't want to be pulling on them, but you want it to be steady. Think of it this way, you want enough pressure that the horse knows there's more contact on the reins and they can't stick their nose out, but not enough that the horse will think you are asking for a stop.
Once you get that all figured out at the walk and trot, and you feel it's VERY consistant. Then move on to the canter. Something I've done with horses that want to "take off" when cantering, is A LOT of transition and smaller circles (thus the need to master the bending before hand). When they start to take of at a canter, bring them down to a walk, and make sure it's done within a few steps (no skid stops here:) just don't let her be putzy about it. Make sure when you ask for a canter, you get it within a few strides, not her trotting into it. If she picks the canter up nicely, keep going, the second that she starts going faster, transitions or circles.
That's just my 2 cents:) Good Luck!! I was in your shoes once, and I think I turned out a pretty good