Under US law the author holds copyright on anything that they write unless they don't for contractual reasons - see works for hire. However copyright is a limited form of ownership.

For instance once you have created and sold a copy, you cannot control the further sales of that copy, or say that certain people cannot own that copy. This is why used bookstores can resell books. (Publishers tried to stop that and failed.) This also covers your ex. However if your ex decided to make copies of your personal letters and send them to friends ridiculing you, then you have a copyright claim against your ex.

Similarly I would be shocked and astonished if mere possession of the physical article of a letter from Ernest Hemingway gave you any right to publish it in your book of letters. This runs counter to everything that I have ever heard on the topic, and before you consider doing something like that, please discuss it with a lawyer! (One of the notable early copyright cases involved the inclusions of letters from Alexander Pope without his permission.)

See this guide on copyright, and there is a direct mention of the case of letters in this primer.

As for the SSSCA, it is now renamed the CBDTPA, and makes half-hearted attempts to address the problems above. What they are coming up with is completely technically unworkable, but now the trick is explaining this to non-technical people who are busy and have no understanding of what the implications of their "small spec change" will be. (Gee, where have we seen this before?) At least they also have realized the potential to create a monopoly position from trying to make it work, and have addressed that. (They also no longer get support from Microsoft for the bill - fancy that.)