Iv'e messed up, please help!

The easiest thing to do is go to a pro.  If you don't want to for whatever reason:

If its white, not yellow, you'd need to go to a beauty supply shop and get a protein filler and use it on only your highlighted hair as directed right before dyeing it a darker shade (because if its white, its going to pick up the base in the dye and it can easily go green or orange, the filler will help prevent that). Then try to dye the highlighted hair only.  I'd suggest a dark blonde, if you still want it to look somewhat highlighted, but if you want to go your natural color, which I'm assuming is darker than that (?)  you can do that too.

If its yellow, you can choose a dye that has a violet base and go over just the highlighted parts with that (don't do that if those parts are white, and try to avoid getting it on your base color (unhighlighted parts) or it could be lifted just enough to make that hair orangy). 

You'll need to get your dye and developer at a beauty supply shop, those dyes will have the base written on them, boxed color at the drugstore won't, so using any of that gives unpredictable results.  10 volume developer would be high enough, since you don't want to lift it more.

Be sure to test strand anything you do...if you don't want to go to a pro to get it dyed over, try to do just your highlighted hair by separating it out. I know that's not that easy, but you don't want to mess up your base color in the process.