Mistake in pathology report

First thing to do is make a complaint to the health service asking for an explanation. You will find a patient feedback form on their website. If you have a copy of the report scan it and attach it. If you don't have a copy, demand one now. By law they must respond to all complaints. Unfortunately, 'respond' is a bit like 'consult' there are huge possible  variations in both action and intention.
Surgeons are adept at robbing off concerns, admitting any liability is a huge professional risk and they will not do it willingly. The health service also has a lot to lose and will not co-operate. ( @allyjay can tell youy how that pans out) Can you sue? Meh, probably not unless you can prove you have been severely compromised. It will take years and truck loads of parasitic lawyers.
 I think the best you can do for now is to officially document the problem and hope that if a mistake was made it doesnt affect your overall outcomes. You can also hope someone gets a kick up the arse, but you will never know about that. Sorry this has happened to you, it is too common and very distressing.

I went  to a public hospital for a second opinion. They were the ones who highlighted the mistake. I have requested for another pathologist to retrieve my slides and have a re-look to see what else could be wrong. Very displeased at the moment.

I'll bet you are. The original provider will still have a complaints  mechanism, but if they are private they are likely to be even more devious. Bastards!

Mine had incorrect year on it. 

I realised that many  breast surgeons don’t really read the path report. They just glance at the last bit. It is up to us to read and ask lots of questions. My breast surgeon had no idea what I was talking about when I pointed out the mistake. And he charged me consultation fee for going to see him too,