Have I really messed up by talking to him about this?

Agree with this 100%

Stop apologising OP, you've done nothing wrong

As you say he was describing what happened to him almost as a third person, keeping it unemotional. You talking about your trauma in probably a more emotional way has stirred up things he thought he had left in the past. If he wants to talk another time let him, but I would keep clear of the topic unless he raises it. He's dealt with it his way, I know people will say he should deal with it 'properly' but I'm not sure that is the best way for everyone.

That is a huge red flag, to me, that he does not know how to be in an equitable relationship. He is comfortable trauma dumping on ^you* but god forbid you should express needs or share your experience with him.

Agree with posters saying you did nothing wrong. He can't seriously expect to talk to you about his trauma, but expect you to realise that talking about yours will trigger him. And he made your story all about him and deliberately mis interpreted your comment. It's completely normal to be glad you've found someone going through something similar, who understands what you're going through etc (which is why support groups for bereavement or certain illnesses exist) without being 'glad' that those people also suffered (bereavement /illness etc). It was a really nasty thing to say because if he knows and loves you, he knows that you wouldn't be the type of person to be glad that he, or any other child, suffered abuse. He clearly said it because it was triggering for him, and led to a flood of emotions. So it's understandable, but it doesn't mean it's acceptable, and he should be apologising for saying this, not leaving it so that you feel bad for not censoring what you said, without any advance request to do so

I think this will turn into a recipe for disasterand you walking on egg shells.

I'd end it now. Neither if you are in a place to be each other's therapist or know what to say. Sounds like both need more work.

It will be a lot of tears if you carry on with him, I can just feel it.

OP you didn't say you were glad he went through it, you said you were glad you had someone who understands you. You did nothing wrong, you are not responsible for his trauma and his reaction.

These are difficult conversations and my sympathies are with both of you. It sounds like he overreacted to your innocent remark and that's probably because of the way he is feeling right now. I think you just need to back away from it and give both of you time to breathe and calm down. You're both feeling upset and vulnerable. Give it some time and park the subject for now. Hopefully when he's had some time, he might feel that he can open up about how he feels again.

Actually I think experiencing violence as a child is very different to experiencing it as an adult. A child has absolutely no option but to put up with it. I was a victim in that way - I dreamed of being rescued but knew that would only happen if I spoke out. I knew that the police would be called, shame would be brought upon my family, my mum would be told I didn't want to live there... I felt so powerless.

He just doesn't want to talk about it which is perfectly valid. It's not always best to dwell and ruminatel on stuff

So the boundary is: he can talk about the trauma he has been through, but you can’t talk about yours as it will trigger him?
I do feel for him.
But I actually feel for you more here, as you are walking on eggshells. You don’t have anything to apologise for.
If he said he was upset and just needed a bit of time, fair enough.
Sounds to me more like he is punishing you for discussing what happened to you.
And if you feel I am being harsh I have been treated for complex PTSD and understand it’s my job to police how it affects me. Just because my trauma, on paper, might seem worse than another person’s doesn’t mean I win a gold in the Suffering Olympics.
Everybody’s experiences are relative to them.
Maybe it’s best not to discuss your ex again, but on the understanding he doesn’t discuss his past either.
I feel you have been both fair and kind here.

I think @DontTouchRoach’s post is a good one. The truth is, you think you’ve dealt with something thoroughly, and then something unexpected can activate it again, and suddenly you’re totally dysregulated and remembering things you didn’t previously.

I experienced SA as a child, felt I’d dealt with it thoroughly years ago in therapy, but only recently the discovery that another family member had been a victim of the same man at the same time was hugely triggering. I kept gagging for days and felt as if there was something going wrong internally, had to call in sick and pull out of a planned weekend away. Plus I’ve found myself remembering things I didn’t previously remember, purely through the other person saying his name and the place. No other details. Just three words.

I think you’re not good for one another on this issue. It’s no one’s fault.

No, he's making himself the centre, making your experience about him. Making his feelings more important than yours. It is unbalanced and displays lack of insight and selfishness. Be careful.

How much has he opened up about his abuse, and has he expected you to listen?

My feeling is that both of you burying your trauma and trying to pretend it didn't happen isn't a good basis for a relationship. It would be one built on sand if both of you are monitoring every interaction, even when something is particularly painful. If you unwittingly did something that reminded him of his trauma, or he did something that your ex used to do what will happen?

You can't force him to deal with things through therapy, but without it he is going to remain damaged by his past. You are going to have to hide who you really are. Can you cope with that?

No I don't think you said anything wrong at all, and I think he is in the wrong for wilfully misunderstanding and using it as an excuse to attack you. Don't let him play those kinds of games.

I'm sorry for what you have both been through.
Your relationship sounds like its becoming unbalanced. You did not behave in an abusive or thoughtless way towards him, and you are not responsible for how he reacts or feels about what happened to you.
Its concerning that you feel responsible for how he feels. You aren't. I think you should talk to a therapist who understands trauma.

He's a dick.

Sorry but I think he sounds hard of comprehension. It doesn’t read anything like a dig at what he went through and I think he’s being a bit self indulgent and turning it into a trauma contest. All you said was that you were glad you found someone who understood you. And you have. He understands you because he had similar experiences. Seems obvious what you meant. Nowhere did you say you were glad that he went through it and he’s really stretching that one a bit. Perhaps this is a wake up call and he does need to address his own trauma if he’s being triggered by completely innocent conversations like this. Are you supposed to walk on eggshells and be afraid to ever discuss your shared experiences in case he blows it out of proportion like this every time?

I said I was on my way to work and didn’t want to talk about it then, but I’d love to chat more to him about it another time as our relationship is fairly serious and it seems an important thing to be open about.

I was quite shocked that he said no. It was too traumatic for him and he couldn’t talk about it. Again he was very abrupt, as if I’d overstepped and triggered his trauma.
I totally understand and respect his boundary of course, but his reaction has made me feel really awful.

I don’t think it’s his fault that his reaction made you feel bad. He told you talking about it had been difficult for him and triggering, and you then indicated that you’d like to talk about it with him again - when he’d literally just told you it had been bad for his mental health. I think I might also have been ‘abrupt’ in his position.

I somehow came away from the conversation feeling guilty by association with my ex husband.

Again, I don’t think your partner is in any way to blame for you feeling like that. He simply said he couldn’t handle conversations about it. Which is fair enough.

Ultimately you’ve both been traumatised by your experiences (as a former victim of domestic violence myself, I hugely sympathise; I’m sorry you both went through it). But that doesn’t mean it’s healthy to act as each other’s therapists, and this probably isn’t an issue to bond over. It sounds like you both have really complex feelings about what happened to each of you and neither of you could ever really be objective in your conversations about it.

Obviously you didn’t mean to upset him and I’m sure things will be OK between you now - but it’s good that he’s explained openly that the conversations are difficult.

This. A partnership works both ways.

I agree with this.

Whether he knows it or not this is a wake up call—he needs to address his childhood trauma before it prevents him from having a normal relationship. He is telling you that there is a “no go” area, for him, and that you are responsible for taking care not to upset him even uf it means not being honest or able to depend on him emotionally. If that is the case it’s something for him to work through not dump on you.