I have fine mid-length naturally light brown hair with golden tones that come up quite light in the sun. I've been getting my hair professionally highlighted for years but the ends of it are really brassy. My roots always come out fine once it has been freshly coloured, but my ends are always a darker brassy colour.
The colour is quite slow to lift in my hair with a pre-lightener and my colourist has been careful to avoid the use of cool tones, ashes, beige blondes on my hair.
I am struggling to figure out what to do to achieve the colour I want, which is a light beige blonde.
This sounds like you need to go and have a good in-depth consultation with a confident colour specialist. From the information you gave me I am going to presume that there is a miscommunication between you and your current hairdresser, so I would recommend getting another opinion.
My top tip for when going to talk to someone about your hair is go armed with pictures, not just from hairdressing magazines but from anywhere that you see someone with the colour that you would love. It is also helpful to take along some images of what you don't like.
It maybe that you are reaching for the unobtainable and a good colourist will manage your expectations to something more realistic.
John Clark, Head Colourist at Brooks+Brooks, London