Sounds like you need to get a consistent forward response from her before working on half halts. If you close your legs around her like you're bear hugging her, she should shoot forward like a tube of toothpaste. The actual half halt is a much more subtle aid, but get the "forward" part down first. Ask her nicely by closing your legs. If she blows you off, pop her behind your leg with your crop to send her forward. Then go back and ask her again nicely with your leg until you're getting a response from her every time. Make sure you're not in her face when you do this exercise. The only goal is forward.
Once you've got the forward down, then you can try half halts. It's worth mentioning the "lazy person's" half halt is a quick trot/walk/trot transition. Do a few dozen of those and gradually decrease the amount of walk steps until you're not quite dropping down to a walk anymore. Each time you do that transition, you should find the reins mysteriously more slack (because her frame has shortened a bit) and you will feel more energy behind you.