AIBU to not understand why he didn't tell me about her?

No.

We're not talking about someone overreacting to the mention of an ex - we're talking about someone being understandably upset and confused that her partner of three years has maintained a friendship with an ex (and supported her through a terminal illness) without ever once mentioning it.

What does them meeting up have to do with anything?

Plenty of people have inappropriate relationships via messages, phone calls etc.

Yet she says in the post above your own
I don't think they've been meeting up

🤔
I don't need to re-read the post, I'm reading it as it is, not how others want it to play out, not everyone on MN has the "He's obviously shagging her" mentality.

So unless he has a cock that can be detached sent, and returned via Royal mail he's done no wrong and there's nothing to defend, and as a side note I can defend who I want without your say so.

did you miss the quote in my post, that I was replying to?

It's the poster I was quoting in my post, not the OP...
which is why I quoted them, to be clear!

I’m sorry I wouldn’t like that at all.
I think that in a serious relationship we would talk about anything.
I don’t think I’d have been jealous but I’d would have liked to know about her.
and even support him in his support for her if you see what I mean.
He lied by omission. That’s not good.

Sorry but I agree with your instincts. It’s a massive lie by omission.
it’s different to having told you and then maybe being in contact more than you thought. This is a complete failure to give fundamental information about who he is and what he is experiencing in his inner life.
It would be over for me.

This is difficult but my gut reaction is something is off here. He is apparently serious about you and wants to marry you but has withheld important information from you. Something isn't adding up is it?!

If there was nothing to hide and he really is serious, then you would be well aware, long before now, of the fact that his ex is his bestie.

No, I think you need to re-read OP's posts and stop trying to defend this bloke.

They were best friends. They were in touch throughout OP's relationship with her partner and he was supporting her through her illness. OP knew nothing about any of that until the lady passed away.

I'd take that she wasn't that much of a best friend then.

There's also a difference between He says she was his best friend, and she is my best friend.

Maybe he was shocked and saddened to hear that she had died and was referring to the friendship from their uni days.

So you’ve been dating a guy for three years who never spoke of his best friend . You never met his best friend either.
to refer to her as best friend implies they were very close and spoke often, probably met up and did things together too … in other words he kept a whole part of his life secret from you - and you from them
he is devious and secretive and god knows what else ! You don’t know him as well as you thought you did .

I think you see your relationship with this man as much more serous than he does .

OP is allowed to 'make it about her' - the person she's been dating for 3 years has lied to her and kept a seemingly important part of his life hidden. She's allowed to have feelings about that!

Agree with a pp - like fuck would I be giving him comfort. His 'friend', his secret, his issue to deal with! I'd be dumping him reassessing this relationship.

There's a huge difference between "not talking about your ex" and not once speaking about your best friend for three entire years

Only, it seems they are in touch regularly, and I wasn't aware of this.

Yet until today he didn't speak of her.

I understand that the ex has only recently come to the OPer's knowledge.
Unless the OPer had asked about his ex and he denied she even existed he hasn't been lying.

Not everyone talks about ex's or feels their past is any business of a present partner, a thread started about the subject on here just recently was in agreement.

I think she means she now realises they had been in touch regularly, not that she doesn't know whether they've been in touch or not.

@theneighbourgotmycake "if someone reacts like that, no wonder their partner wouldn't mention their ex, ever!"

How to miss the point. She's reacting like this precisely because he's never mentioned the existence of this "best friend". Yet he was in regular contact with her. Besides, I hardly think her reaction is over the top.

We don't actually know of they were in touch. Op said "seemingly in touch" which suggest she has no clue whether that were or weren't. Parents called, so what. If I die, my parents are going to call my childhood friends many of whom have not changed their phone number in 25 years. I also know a lot about their health and circumstances because they post about it on SM even when I haven't spoken to them for years. There are a lot of bloody assumptions on here. It must be hard living your life smelling a rat in every relationship...

Did you miss the bit where she's referring to him LYING for three years?

if someone reacts like that, no wonder their partner wouldn't mention their ex, ever!

Do you know what 'potentially' means? And no I haven't got my evidence just assumptions a d opinions which is allowed on mumsnet last time I checked

The fact it was a secret relationship doesn’t look great.

Whenever there are threads on here about male/female friendships, I’m always on the side of thinking these are totally fine. My best friend is a man! But he’s not a secret from DH.

Yes, but that's not the case here. The OP's partner was in regular touch with this woman for the past three years. He's devastated by her death because, as he says, she was his (current) best friend. He was also phoned by a close family member of the deceased to inform him of her death, so his number was obviously known to them or easily accessed from the woman's phone. So this isn't some long ago faded connection.

Like fuck would I be supporting my partner after his ex fuck buddy he’d been keeping a secret for 3 entire years. How has it ever been mentioned even in passing that they were still in touch, let alone she was literally his best friend??

That'll be the 'kangaroo court' of MN sheep says so.