Horses are creatures of habit. Your mare is not "having a tantrum" like a child would, but acting out of the habit she's been allowed and taught - that the field is meant to be galloped through in a specific direction. Your goal now is to reprogram this habit and mix things up a little, so that she learns to listen to you, not to just follow the program she's been conditioned to always follow.
If she's prone to "bomb off", I'd suggest starting out with walking here there in hand. Instead the field being a gallop zone, show her it can be a relaxation zone - walk her quietly up and down the field many times, until she's completely calm in all spots of it, let her graze, do a bit of work in hand - lunging at the walk and trot, yielding her hindquarters, stretching and flexing softly. Let her think and relax.
When she's good in the hand, start riding, if possible - with another horse&rider to accompany you. Start off at a walk and only trot if she's truly calm and responsive. Whenever she starts speeding up or aiming at home, ride her in circles, working on a good, soft lateral flexion throughout her whole body, yield her HQ, if it's in your repertoire - do some shoulder-in, leg yielding and other lateral exercises that make her think and concentrate on your ques.
I would strongly suggest that you forget about cantering and galloping in these fields until she is completely calm and responsive at the walk and the trot, as well as stopping softly and changing directions away from home whenever you ask. You may, however, walk back home, canter her in the arena, then walk back to the field - thus, the fields become a place to relax, and home - a place to work, making it less appealing to just bomb homewards. Good luck!
I have come a long way, to surrender my shadow to the shadow of my horse.