It sounds like you've duly considered and investigated the possibility that your attending may be right and you wrong, and have still come to the conclusion that you are right. It also sounds like you have tried to alert your attending of that, and the attending still does not agree.
Do you still believe you are right?
If yes, then you are obligated to take it another step.
As physicians, our first obligation is to our patients - not to our attendings, not to ourselves. If you really think you are right, and you have good evidence to support your position, you owe it to your patient to stick your neck out. Just imagine if it were your grandma, your child, or you, who would be subjected to a mistaken diagnosis and inappropriate treatment.
That said, you don't need to make a federal case out of it. My advice would be to gather all your slides, formulate your story and opinion and be ready to summarize it succinctly, then take it all to a senior pathologist or attending ally and state your case.
It will be awkward and difficult. But, if you really think you are correct, you have no choice.
I wish you well.