How to keep my pony's head up from the gr when leading?

i am a 12 year old girl and my pony is 11.2 hh and incredible strong and SUPER fat after a year without riding and she has lost all her manners. so when im leading her from her field to her stable she drags me around and pulls me wherever she fancys to eat gr which is dangerous since we have to cross a road outside her field, ive tried being tough and in control but she takes advantage and i need to go fech the bridle or get my dad to lead her. i would really aprecitate advice on this so please anser.