Leading a 2 year old?

We have a 2 year filly, We''ve been working on ground work with her,longing,she's had a saddle and rider on her for a few minutes. The most problem that we've had is leading her into the round pen. She is boarded out,but its stall board with limited turnout. We are trying a rope halter on her now,she is very strong and once she puts her head down to graze,its almost impossible to get her going again.Then she''ll try to run you over. Any help would be appreciated.