GP's dismissing my complaints about a breast problem for nearly 4 years

When I went to my GP (slight pinkness and  tenderness), she had a look then uttered the best possible line, " We need someone who knows more about breasts than I do".

GPs can't know everything and the most likely cause may be the correct one. It's also possible that you had a physical reaction to minute changes early on, given that a mammogram and ultrasounds failed to detect anything - which would reassure most. Good for you for keeping at it, but do tell those GPs what has happened, as it will inform them in the future. Best wishes.