Hello there,
I strongly suggest you find a colour specialist salon and look for someone with experience. I have many readers who have experienced similar situations after using dark brown hair colour for a long period. The p-Phenylenediamine, or PPD for short, contained in dark colours is difficult to shift - many people also get skin reactions from over use of dark hair colours.
A light brown or dark blonde colour applied by a professional colourist should not be too much of a problem. The ends of your hair will probably need lifting, as long this is done carefully and good conditioning products are used the light brown tint should take well.
However colours do wash out unless the hair is in tip-top condition. Let your colourist recommend a special colour restore shampoo to use at home, and maybe you will need a colour conditioner or treatment to keep your hair cuticles closed. This will keep the new colour locked in and stop it from washing out.
I always encourage my clients to bring pictures of colours and styles they especially like, it is a good way to communicate and leaves out misunderstanding between your stylist/colourist and yourself. Compromises need to be accepted at times depending on your hair structure and facial features, skin tone etc.
I do hope that my advice will help you and good luck with your new colour.