Thinking of you and hope your appointment goes well today!
I think it's normal to doubt what you have found and 90% of breast clinic visits do not result in a diagnosis of cancer. Your GP wouldn't refer you if they felt it was unnecessary but equally there's probably a good chance that it is a benign condition too.
I hope you are coping ok and that things go well!
ps. well done for being so brave and going by yourself. I hope your Mum's op goes well and that your sis starts feeling better soon too.