Make sure you keep her busy. Do lots of circles, serpintines, shoulder in, turn on the forehand, shallow loops, changes of rein ect. This will keep her thinking and keep her interested and keep her mind on you not bolting off. Also if possible try riding alone, as this will stop her from running to other horses. When she pulls make sure you half halt (squeeze and give on the outside rein only) Instead of just fighting and pulling against her. When she does bolt off, put her on a small circle and make her canter round this a couple of times as if it is kept small she will not be able to rush off, and she will soon get bored of the idea of rushing off as she knows she will have to do the circles. If all else fails, you could consider changing the bit or putting her on a low energy feed? Hope I've helped.