How to make my hair lighter?

i have naturally blonde hair, but i dyed it really light blonde a year or so ago, with brown, and then black underneath, and stupid me used a permanent black, so i killed my hair sooo bad dying it back all blonde again (first time i had to bleach my hair, ever!), now that small underneath layer is really really brittle, broken and dead, and just as long as my neck, but the rest of my hair is a few inches past my shoulders, so its annoying. But anyway my regrowth is really dark brown? And every time i dye my hair, it doesn't seem to get lighter, only for that night then it goes back to being a yellow-ish kind of blonde, it just won't go lighter and i really want hell light bleach blonde hair! Its nearly there, but not quite. I've been trying for so long. Any ideas?