@Jream
Understanding why you don’t want to tell anyone may be key. Sometimes it’s privacy (my life, my body, nobody else’s business), sometimes it’s a misplaced sense of shame (did I do something to cause this, which you almost certainly didn’t), sometimes it’s fear of people treating you differently.
Whichever, who you tell and when is entirely up to you. I’m a big fan of the qualified outsider (not a friend or family member) who can help you work out your feelings - the brain is just as important as the body in this caper.
A cautionary note - not a personal experience, but it’s by no means unusual for friends and family to need some comforting from you about your diagnosis rather than the more predictable other way round. Cancer is just one more serious illness but it’s been a bogeyman for so long that talking about it can be difficult. So yes, a professional counsellor to practice with! Your GP, oncologist or breast care nurse should be able to provide a referral. Best wishes, you’re on a new unknown path but the outcomes (I am 10 years on and no evidence of disease) can be wonderful.