What is the word for 'saying more than you intended'?

Merriam-Webster does not even flag this as informal, but surely it is:

blab [verb] [intransitive]

...

2: to reveal a secret especially by indiscreet chatter

[transitive]

to reveal especially without reserve or discretion

blabbed the whole affair to the press

Cambridge English Dictionary adds the 'informal' tag and gives a more closely matching definition:

blab verb [ I or T ] informal:

to talk carelessly or too much, often telling others something you should keep secret....

The phenomenon is called Freudian slip.

Freudian slip: A Freudian slip, also called parapraxis, is an error in speech, memory, or physical action that occurs due to the interference of an unconscious subdued wish or internal train of thought. [Wikipedia]

Or Parapraxis.

Parapraxis: a slip of the tongue or pen, forgetfulness, misplacement of objects, or other error thought to reveal unconscious wishes or attitudes. [Dictionary.com]

Or Lapsus.

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If you need a verb for the action of saying more than intended, then I suggest blurt.

Blurt (out): To utter suddenly or inadvertently; divulge impulsively or unadvisedly (usually followed by out).

Example: He blurted out the hiding place of the spy. [Dictionary.com]

Or spill the beans also fit the description.

I remembered Skid talk from an essay in an old English (UK-published) high school literature book we had lying around. The piece was supposed to be representative of American literature.

Skid-talk is more than a slip of the tongue. It's a slip of the whole mind. In effect, it puts one idea on top of another, producing a sort of mental double exposure - and my friend Bunny is a master of the art. When her husband, a prominent Hollywood director, completed a screen epic recently she told him loyally, "I hope it goes over with a crash." She was very enthusiastic after the preview. "It's a great picture," she assured everyone. "Don't miss it if you can." -- Corey Ford, published in RD, 1954 sometime. [Truth-unleashed Blog]

It seems to have been a one-off; I can't seem to find it in any dictionary, not even American ones, and a quick online search only brings up references to the original article or derivative works. However, it is sure enough an American classic, so here it is for what it's worth.

In American military propaganda, the term "Loose lips" was used to refer to accidentally revealing secret information that an enemy could use against them, the full phrase being Loose lips sink ships.

When you reveal something inadvertently it is (from Collins)

let slip
_PHRASE

If you let slip information, you accidentally tell it to someone, when you wanted to keep it secret.

Example: I bet he let slip that I'd gone to America.

A slip of the tongue is something that is said by mistake. It normally describes a small, short word or phrase that's said by mistake while trying to say something else. It can often occur when someone is thinking about something other than what they're speaking about. A slip of the tongue is often recognized and corrected immediately by the speaker as they realize what they've said.

I would use 'overspoke'. The context would preclude meanings related to 'exaggerate' or 'shouted over'. I think it would make the point, as in "Perhaps I overspoke..."

Idiomatically, running off at the mouth.

run off at the mouth

1. To speak without discretion; to speak too loudly or freely, especially about sensitive topics or information. We would have gotten away with our plan if your dumb cousin hadn't started running off at the mouth all over town. thefreedictionary.com

I'll nominate the relative newcomer "overshare." According to Merriam-Webster:

over·​share | ˌō-vər-ˈsher overshared; oversharing; overshares Definition of overshare transitive + intransitive

: to share or reveal too much information

Shakarian cautions students to not overshare information online, to create complex passwords and to only use secure networks. — Emily Giordano

The sharing and discussion of emotion has always been heavily gendered and women who "overshare" details of their private lives have historically been maligned. — Rachel Sykes

… [Rihanna] doesn't overshare her life on social media — despite having 60 million and 86 million followers on Instagram and Twitter respectively — and you never know who she's dating, despite who you see her with. — Joshua Eferighe

In my experience, it's used more often in cases where the result is social discomfort rather than a security breach, but the above examples show that it can be used across the spectrum.

The real answer is simply "[... but I've] said too much", but there is a common English expression "to let the cat out of the bag", a metaphor since it literally refers to revealing too much.