To think, 17 years on, my Mum could let this go?

Your actions could have saved the moany cow from financial ruin!

I think your subconscious, not your conscious mind, had already noted some furtiveness and oddness round your father and his finances. People get furtive round their finances when they are doing something dishonest - whether cheating their spouse or HMRC and your subconscious was probably chewing that over. Your mother though is an ungrateful cow. Your father is the one who had the affair and was the one with the "cold, calculating and generally untrustworthy nature!" That's the man she chose to marry and what difference would it have made if she'd known a few weeks earlier. It doesn't sound like she would have had the commonsense to download the financial information. Your mother should be exceedingly grateful rather than railing like a petulant child.

I have to say i would say very sharply don’t worry mum, if I ever again have the opportunity to save you from being financially screwed over, I will step firmly back and do nothing, I’ve learnt my lesson from your constant accusations despite my saving your bacon with that file.

Your mum is completely unreasonable, illogical and unfair, as you know.

Her issue is that she needs someone to lash out at, and since she can't lash out at your dad because he's long gone, lashing out at you is easier than blaming herself.

Affairs are rarely "out of the blue" and she knows deep down there were signs, there always are. She just can't admit it to herself.

Perhaps you need to have it out with her once and for all. Tell her bluntly that if she doesn't stop, she could lose you too.

You have amazing instincts OP, and your Mum should be thanking you forevermore!
I can't imagine why she's behaved in this way for so many years, just some people are weird and focus on the wrong things/people to blame when things go bad.

Your mothers ungrateful and bonkers attitude towards you clearly explains why your father had an affair and left her.

I can see why she’s suspicious, and subconsciously maybe you did know something was going on, even though you couldn’t get your head around it or articulate it at the time.

Either way she certainly shouldn’t still be giving you a hard time about it after this many years!

I think what you did on instinct was impressive. She should be forever grateful to you for having had the nous that she, as a mature adult, didn't have. I would be telling her this straight in a respectful but firm way. If she says it again, calmly repeat it, each and every time.

You had an odd feeling, in other words, @HowSharperThan— and I completely understand that.

I was lying on my parents bed, watching their crappy little black & white tv - because whatever everybody else was watching in the other room I didn’t like.
So my dad comes in, gets in his wallet on top of his dresser and pulls out a note - and read it under the light of the lamp. I wasn’t paying any attention until he laughed this dirty, leery, laugh. Only I didn’t know that’s what it was - just that I’d never heard it before and it was off - it gave me an odd feeling.
So for the first and only time in my entire life I looked in Daddy’s wallet. And the note said ”I’ll have drinks with you but I won’t park with you.”
I was sick. I wrote him a note and told him I wouldn’t tell Mum unless he didn’t, and that since. It was 2 weeks until sisters wedding, I’d give him until after that.
Then he chased me down and tailgated me while I had my grandfather in the car. I was terrified of arriving at my Papa’s barbar shop, where I took him every 2 weeks.
So I caught my dad and he went insane.
We called a truce at sisters wedding, which means I had to apologise.
Christ.

It is not up to the child to explain anything other than leave 'me out of it'

It is not up to anyones child to be involved in their parents relationship, even if a child knows it is up to the parents not the child

Even if you knew your father is to blame for putting you in that situation (or mother if they were cheating) you are not to blame and I know going NC is the in thing on here I would be sayong you mention it to me again I will be going NC

Adult children or not why do parents think their children are counsellors?

At this point I'd tell her to drop it. You were just 17, FFS. You had not understanding of what was going on. Your mother's anger us significantly misplaced and she is using you as the target when she has never gotten iver the betrayal of her husband. I'd be fed up, tbh. I'd tell her to shut it and get counselling unless she wants to destroy the relationship with you.
Please don't take on any responsibility for her emotions and suspicions and if you have, get counselling yourself.

Your mum sounds like work and what you described her sheds light on why your parents divorced.

How did you get all the information downloaded? Surely financial information would require access to protected systems, unless your dad weirdly filed a load of scanned-in documents in unencrypted folders, in a very orderly fashion.

It is such a bizarre thing to have successfully done, I can well understand why your mum still puzzles over it. Obviously it helped her and she should let it go, but I think incredulity would be quite understandable -not cruelty or talking down to you, that’s definitely not acceptable.

"No you're right mum. I too wish I hadn't helped you back then as you've done nothing but give me grief and treat me with suspcison since - both of which are massively unfair when it was him who hurt you."

On repeat every time she mentions it.

You’re mum understandably has trust issues. Have you ever explained it this way to her?

Wow! What a clever, smart, intuitive seventeen year old you were. I am
in awe.
If I were your mother I’d never stop thanking you and hailing you as the hero of the hour. I’d feel indebted to you.
As is sometimes the way, wrong mother for a great child. You are wasted on her.
I don’t know how to say this without sounding disrespectful of your mother but, she needs a good boot where the sun don’t shine.
If you were mine, you’d be sick of basking in the praise.

This. You're in the position of the best friend who tells her mate her "D"H is shagging someone else and is then blamed. Shoot the messenger.

This ^
She should be profoundly grateful to you all these 17 years.

Have you explained what really happened? (I can imagine that you have done thousands of times!). I think she's displacing the sense of betrayal she felt from your Dad and putting it on you, which must feel even worse when you actually really saved her!

Sorry to be blunt but is your mum a bit… dim? If so that is why she’s suspicious of you and sadly she probably always will be. My mother had a TBI that impacted her intellect and has always been suspicious me/my intelligence and quick to accuse me of things, particularly when I was a teenager.

OP I think your mum holds this against you because you are far more intelligent than she is and she doesnt like it. You saved her bacon and she doesn't want to admit it.

If I were her, I’d keep bringing it up too. To thank you for being such an amazing daughter and saving our bacon. Honestly she should be eternally grateful to you.

It sounds mad to me. I can see why she thinks it's incredulous that you just randomly happened to decide to do that.

Spur of the moment curiosity is one thing, but to sneak downstairs to do it? Then to forget about it too? Do people even have files and files of financial information stored in their computers? I certainly don't. That might be why I'm so poor!

Yeah I think I'd be suspicious of you too. It's just such a coincidence if you didn't know. Of course it was helpful too, and she ought to be grateful that she had such a cunning daughter.

Maybe in future go with "I felt something was off and I acted to protect us. I had no facts, just a feeling that he was hiding something. That information was only helpful if my intuition was correct and there was something to worry about. Had dad not been dishonest himself then I wouldn't have had to use it. It wasn't my secret that hurt you, it was his."

Maybe that would shut her up?

Sometimes you can know of something intuitively long before you 'know'. On some level you probably knew something was amiss, that simply means you have a good strong intuition and it happened to be correct. Of course that doesn't mean you factually knew his secrets enough to tell another family member. She is so wrong here OP, it's really poor form that she doesn't believe her own daughter. I don't think you can do anything to convince her otherwise. It makes me wonder was she the type to wrongly accuse her partner of cheating for so long that in the end he said he might as well.

If your mum is a reader, give her a copy of Thinking Fast and Slow. This sort of quick, instinctive thought is well attested. She should be thanking you.

She should be absolutely thankful to you and thanking you forever more! Not criticising you.

So maybe she shouldn’t let it go - in the sense she should forever see you as the one who saved the day!

Awful that she calls you deceitful and accuses you of keeping secrets. How downright nasty of her.