Though we totally get your frustration when you are going to a professional and not getting the results you want, it is usually much worse off when you take it into your own hands. Your color process (as you already know) is not straightforward. Color fillers come in a large variety and help to achieve different results. Which color filler is used is based on the results of the lightening process on your hair. There are many many variables.
In short, our best advice is to ask around and seek a professional that will get you the best results while maintaining the health of your hair. They are out there!