I reached out to someone who runs a niche website two times about a sad personal matter that fits their niche and after being ignored two times I sent an email where I blew up at the individual for ignoring me. Several (niche) celebrities were "collateral damage" in my little explosion. The niche website owner then posted my emails online and embarrassed not only me but the celebrities involved, who somehow know about my emails (maybe she sent screenshots to them)? and the celebrities made an official video to me, angrily talking about what I said. Having a false impression of me now (me at Final Straw VS me at All Other Times) the celebrities now hate me and have released official copyrighted forms of shall we say "entertainment media" disrespecting me specifically, and insulting me and things unique to myself (body type being one specific). This wouldn't have happened without my private Gmails being (screenshotted?) and posted online by this person, so am I legally able to sue her? If so what could damages could I ask for based on what's happened? And am I likely to win my case and what branch of attorney am I looking for with a situation like this? I'd understand if I posted something online and had problems because of it but these were private emails between sender and recipient.
At what point did you tell them the correspondence was confidential?
Unless and until you do, it isn’t. Some relationships (e.g. doctor/patient, lawyer/client) default to confidential but most do not. Ever wondered about those standard blurbs at the bottom of corporate emails that mention confidentiality- now you know why they are there.
However, even if you do tell someone something in confidence, that does not make non-confidential information confidential. For example, the name of the president of the United States is not confidential information even if you mark a letter to them as "confidential".
Even if these were confidential you are only entitled to damages. So, what damage have you suffered? At law, hurt feelings and wounded social standing are not "damages" - you have to be able to put a legitimate cash value on them. For example, if your boss fires you because of this, then you've suffered damage. Like defamation cases, breach of confidence cases can end with the plaintiff winning, being awarded $1 and being stuck with the legal fees.
Further, if you defamed these “celebrities” then they can sue you.