Answer

By Your Horse

Is she getting enough food? Being hungry makes anyone grumpy! As your horse is still growing, she needs the best quality ad-lib hay/haylage and minerals (rather than hard feed), and also needs daily turnout to strengthen and exercise her joints and ligaments. Social companionship with other horses is essential, and often helps with discipline as she will be put in her place by others.
Once all of these measures are in place, you can begin to address the specific problem. She may have lost out while living with another horse and now safeguards her food too zealously. If so, in time she should become more relaxed. She must acknowledge you as herd leader, so never show fear or anger when she kicks out at you, just firmly move her away. To do this safely initially, you will need to have control of her in a headcollar or pressure halter.
Devote time to some groundwork and discipline outside feed times too. Working in a small arena, practise moving your horse away from you with a gently swinging rope. Place a feed bucket in the arena and move her away from it until she shows submission, then let her approach you. Doing this in a larger area means she won’t feel pressured and you can read each others’ body language. Remain calm and confident at all times, showing her the leadership she needs, rewarding good behaviour and allowing her to rest and chill out. When she’s resting, stroke her all over with long, rhythmical strokes.