how to go darker

Based on the sort of question you're asking and the complexity, I think it'd be wise to seek out in person a trusted hairdresser whose expert in color changes.

Firstly, why would you take such a risk of messing it up by attempting it yourself?

Secondly, why would you trust total strangers with advice like this? I don't think that is a wise idea.

Lastly, I could be in error, but I know of no professional hair colorists here on A2K.

What you need is a toner to remove the brassy color.

Find a blue/purple toner. You can get those easily at a sallys store. I would say go with the purple to get rid of the brassy color the best and to even out the color you have now.
Then get a dark protein filler for the rest of your hair and add it to your regular shampoo , or just put it in your hair before you shower. Try not to get it into your highlights though.. it wont remove them exactly, but it can cause them to tint slightly.

If the only problem is the unbalanced brassy color, a toner will fix that and you will have the color you wanted in the first place.
im surprised your hairdresser didnt inform you of that first...
Having more than one color in your head requires a bit of home maintenance to keep up with.

I know.. I have black hair and a 3 inch thick white streak. It takes work every week.

You HAVE to use a toner every 2 weeks or so to keep that color balanced and keep away the brassy color. Just using a purple tone conditioner isnt enough.