Can I sue my partner/SO for invasion of privacy?

Sue her for what? You have suffered no loss from the reading of the letter. You suffered a loss because you were discovered to be having an affair.

Traditional common law privacy law includes a tort of intrusion into personal solitude. The intrusion itself is the harm and no publication is required.

One of the key questions in this circumstance is whether one has an expectation of privacy in one’s computerized information against a spouse.

There’s another cause of action in privacy law covering publication of private facts. Truth is not a defense.

The other privacy torts are painting someone in a false light and misappropriation of one’s identity. Those might not be relevant for this.

There’s also the Computer Fraud and Abuse Act, which prohibits accessing someone’s computer without authorization.

The OP was about a letter in a pocket, not computerized information.

Right but how much harm is that intrusion worth?

You may be able to sue for the intrusion into your personal solitude, but you can’t, IMHO, sue for the divorce and loss of career.

Do you have some basis for that opinion or is it just a bare opinion?

Every opinion I have ever had has been based on something. Every single one.

But seriously, what part of my statement do you disagree with?

I didn’t say I disagreed. I want to know what the basis of your opinion is. How do you know that those consequences can’t be recovered in a privacy action? How does that compared to what could be recovered in a defamation action?

What consequences?

But can you call spirits from the vasty deep?

I’m really not asking riddles here. The consequences you mention in your post. Those are harms. They are harms that very well may be the foreseeable consequences of invasion of privacy. Why do you think that they aren’t recoverable?

Because I don’t believe she is legally the cause of his harm.

His harm was caused by his cheating. How it was discovered is I think, irrelevant.

Are you claiming that she should have known that when she looked at a paper found in her drunken husbands pocket, she should have foreseen that the paper might be evidence of his infidelity? And that because she found the paper while “invading his privacy”, it is now she, and not he, who is responsible for any and all damages resulting from said infidelity?

You’re just restating your belief. Is it based on some actual knowledge of this area of the law? What I’m trying to get at is the basis for your belief? Is it based on knowledge or are you just guessing or saying what you think should be true?