What are your advice on how to stop a horse refusing?
Lots and lots of work. All of it actually boils down to holes in your foundation (ground work). I personally spend a lot of time with trot poles and small verticals (2-3ft) and bounces help too. Just keep in mind you don't want to develop a rusher (harsh hands are the cause of a rusher), so be sure to be confident but effective.
What is the best bit for a horse that lean on your hands almost pulling you down? She has gotten better at the trot but cantering is a constant pull and I can't collect her well.
I agree with Sofia but I've HEARD that waterford bits help with this. Again I've only heard by mouth and never seen it with my eyes so I can't guarantee. I would do some more research before buying one anyways :)