Will she back up? I had the same problem with a colt last year so instead of trying to make him walk on I made him back up instead. The idea is keep backing her up even when she's had enough - then it becomes a far more attractive idea to go forwards!
You don't say when she does this. Are you doing much work with her? If so, it may be her way of telling you she's had enough for a while; remember she's very young and she's maybe getting a bit stale. Perhaps a break of a few days would help if this is the case.
You could also try "leading" her loose in the field, if you can. Have a bribe in your pocket and see if she'll follow you for it, without a lead rope. Make a big fuss of her if she does and she'll be more likely to do it again. If she'll happily follow you loose, it's a big step towards leading her off a halter. When you do get her on a halter, try not to get into pulling contests with her - she will always win. Let her choose when to move forward (you might need lots of patience!) and then reward her straight away. Encourage her with a bribe by all means, till she gets the iea that leading is more fun than being awkward.
Hope all this is of some help - they're all methods tried and tested on my youngsters. Let us know how you get on!