Acheiving platinum... help!

a few months ago i asked my hairdresser to make my dyed brown hair platinum. She got it pretty much platinum at the front but the rest was light ash blonde. About 2 weeks ago i decided to attempt getting my whole head platinum myself. I used schwarzkopf xxl live in absolut platinum. It lightened my hair but left alot of brassiness. I used the same dye again last week hoping it would only get whiter.Its lightened my hair but theres alot of orange/yellow tones left, especially underneath. Since then i've used wella shaders and toners in light ash blonde to try and remove the brassiness but theres no difference. My roots are coming through and im left with a mess of colours. Even if the brassiness is removed i doubt i'll be left with the nearly white colour i want. I asked in sallys about toners but the assisant was of no help whatsoever. should i just bleach my whole head? or use a strong toner?  Please help! my hair is really getting me down, all suggestions would be greatly appreciated, as i have no clue what to do next.