How would you handle what happened at the work Christmas do?

You're not a grass. It was incredibly unprofessional of her to discuss with other people and ultimately if she can't handle her drink and behave then she should not drink / limit her drinking.
She owes you a formal apology at the very least.

She should never have discussed a work error with you at a Xmas party or with others whatever your error was hers is far worse ,you were out of work time completely inappropriate of her

She was very very drunk and probably feels mortified now, if she even remembers what she said. I can't see that anyone overhearing the 'conversation' would think badly of you. Even if they weren't aware of your mistake, literally everyone makes mistakes at times, and I bet most of us have sent the wrong email or information to the wrong person at least once, so it's not a shocking thing to reveal, though it should have been kept confidential. Try to let it go.

On a company night out your drunken senior colleague repeatedly humiliated you and then proceeds to repeat your confidential information to other staff?

I think you are hugely under reacting.
Fxxk that. You made a mistake, it was dealt with.
Is it company policy to repeatedly humiliate staff?
I would be absolutely apoplectic.
Who the hell does she think she is?
I have heard of people being severely reprimanded for less.

You are entitled to dignity in the work place.
You are entitled to feel safe in your work place and on a night out.

Neither of which you feel now.
Find your anger.

A formal complaint. Use confidentiality as the main reason. I assume it was an event not on company premises, so that someone working there not part of the company (say bar staff) could have overheard.

Your manager is supporting you and good that you now know that the matter is closed regarding the error. I bet there is an issue with this member of staff that maybe you’re not aware of.

If she had kept the conversation between you and went on and on, I'd have waited till she was sober and bring it up, expecting her to be a little embarassed for drunkenly being so rude, and would expect an apology and I'd be ok to leave it there.

As she brought in other colleagues and was badmouthing you, I'd find that inappropriate enough to take it down as a formal complaint.

To discuss it with a colleague to ask an opinion i.e. "Sneaky messed up by sending this email with this info in, what should I do to fix it?"- absolutely fine, apppropriate and not finger pointing. Drunkenly disrespecting an error you made months ago suggests she has more of an issue with you than the error, and she needs to be dealt with.

You did the right thing to speak to your line manager. This manager is bullying you by bringing up your past mistake and deliberately humiliating you then discussing it with other employees. Being drunk is no excuse for what she did - it’s horrible and completely unprofessional. I’m quite feisty so the bitch would have had a drink ‘accidentally’ tipped over her!

Go to HR as soon as you can. That is an absolute shit show and you need to make your position clear right now.

Your manager is 100% right. You need to raise a grievance. This manager should never have discussed your work with someone else. How dare she!!

how do you usually get on with this person?

ignore!

You spoke to your manager who I would have assumed would rather it was all forgotten about so they can get back to the other 100 things on their To Do list. The fact that your manager has encouraged you to make it formal indicates it's probably a good idea to do so. I'd have a clear idea (in your head at least) of what you want to get out of it, so the process doesn't drag on longer than you'd want.

Exactly - she was completely unprofessional

Make a formal complaint. She’s been highly unprofessional. She’s a manager and should behave like one.

Seriously unprofessional of a manager to get so drunk and then breach confidentiality. Your line manager is correct and the formal route will be best. She will prob get a warning but needs to understand how serious this is. Might not realise if it was handled informally.

I used quotation marks to quote OP in the post immediately above mine. Which I presume you just read.

I am glad you spoke to your LM and they are taking it further.
She was extremely unprofessional, we have all made that kind of mistake before .
(The fact that you had to go through additional training surprises me)
The fact that she brought it up 6 months later is really poor on her part.
Even bringing it up a week after it happened would be a bit off.
Even worse that she started telling others at the party about it shows a real lack of management skills .

It is only grassing if you dob in a peer. She is a manager.

Just have a quiet word with your line manager, nothing formal, if your departments have got a good relationship they may have an informal word, but to be honest I bet that other manager is feeling the guilt, and in there drunk head although inappropriate may just have been an anecdote, it’s wasn’t pleasant for you but may even work in your favour, but formally complaining probably won’t

YANBU,

The actions of the idiot manager at your xmas work do was to cause you unnecessary alarm and distress which she needs a reprimand for. The manager should be told to apologise to you for her absalutely atrocious conduct, she should be totally ashamed of herself.

You may need to get hr involved.

Remember, she shame is hers' OP not yours.

What she did was bang out of order. I am in a leadership role and I never have too much to drink at these things, it’s not worth it.

I would report it formally as Teams change and if she is in the management and you may be managed by her in future. This has all to be documented. Managers report everything formally why you can't?

Ah the joys of the office Christmas outing. If it's any consolation Op she's probably mortified already, if she can remember what she said, of course.

Everyone is probably think 'there but for the grace of god'. Meaning most people have made some mistake in their job (including this manager).
She's made an idiot of herself. Report it to your own manager.