Can (should) I get over this?

[Drowningincokezero]

So, if you have a dig about my previous posts you'll see a nice little story developing whereby I initially had a sense of unease about my partner (of 3 years now) having a possibly inappropriate friendship with a female work colleague 20 years younger than me (I'm 48) and 27 years younger than him. I'd brought it to his attention that it made me uncomfortable, but I didn't quite know why, and would prefer him to stop their connections on social media. He'd said that as he'd been in controlling relationships before, he wasn't prepared to cut her off like that and that it would have been difficult to explain to her why he had done that when he needed to keep a good working relationship with her. He worked there for just over a year and I sat on this feeling, occasionally bringing it up only to be told all was fine and nothing to worry about. He even suggested counselling for my paranoia, which I didn't take up as my life is just so busy and I really did start to think I was just being stupid. I got the impression that their friendship had turned sour about 3 months before he finished with that company for other, work-based reasons. I then pointed out that he doesn't have to worry about offending her anymore, and he did then unfriend her from SM and that was that.
2 years later and he's about to move in with me and I had one look at his iPad as this had never sat right with me, and it was coming crunch time with the moving in coming soon. I wanted to be absolutely sure about this. Anyway, lo and behold there are the messages, basically innuendoes and lots of comments on her beauty etc. I confronted him and he was adamant that it was the work culture at that firm and not out of place with what everyone was saying, it was only ever banter and everyone knew it. This has been backed up by another employee there that I do trust, who greatly derides the whole culture but agrees that it's just the way that employees would interact there and that innuendoes were just jokes.
But these were private messages! When I say this to him he says yes but they would have been triggered by a conversation previous that day that would've been overheard by others and that it was meant and taken as a joke. His behaviour towards me has otherwise always been very good. He comes across as committed and very loving and demonstrative. I know he's got a wicked sense of humor and would indeed find all of this funny. But I can't get past it.
How would you feel about this and deal with it? He's remained open to talk about it with me although doesn't like to as he says he knows what he did was hurtful and it makes him feel like an arsehole each time all over again. I have talked extensively with him about it, nonetheless.
I think I class it as cheating, regardless of the intention. Just him even writing those flirtatious words breaks my heart and, in all honesty, my love for him. I've become totally preoccupied with this and it's affecting how I relate to him now. He would like to make it work and has to date shown lots of commitment in uprooting himself to come and be with me (we have been long distance, with a few months at a time spent together more recently). I would just like to go back to how I felt about him before, when we were so in love and, I thought, besotted with each other. He says he has never waivered in his feelings towards me and if he'd wanted to pursue this woman then he would have, but he didn't.
It's got to a point now where I have to put it behind me because it's breaking the last of us, there's no new ground being covered and it's pushing us further apart. I'm at a loss as to what I need to do.

Edited for typos

I couldn’t forgive this. A man who loved a woman wouldn’t behave like this with another women irrespective of the culture. It’s disrespectful. And the fact that he kept calling you paranoid and suggested you needed therapy when your instincts were absolutely correct is a very manipulative form of gaslighting. Gaslighting is emotionally abusive. I wouldn’t continue on in a relationship with a man whom was a proven liar and disrespectful gaslighter. I’m sorry this happened to you but you’re worth more than that.

he’s twice her age . that’s really icky tbh

I don’t think it matters whether or not it is part of the work culture. I doubt EVERY member of staff joins in with this.
It’s something I’m guessing you wouldn't do if roles were reversed and therefore this is why it is making you feel unhappy and eroding your feelings/creating distance.
I’d feel the same in your shoes. The huge age gap and I imagine power imbalance in their roles SHOULD mean that he’d rather shelter her from this sort of attention (actually its flirtation) than enter into joining in with it.
The fact that he hasn’t and has actually continued it into PM’s would give me a serious rethink of the whole relationship. I mean, its not necessary to the job, so there’s no need for it, except for a tacky little ego boost.

not to mention totally inappropriate and unprofessional. My respect for him would be lost .

Also…theres a reason it makes him feel like an asshole when you talk about it. Because he is one.

He’s betrayed you, and he’s not sorry, and he’s trying to frame it as a failing in you.

It would 100% be the end for me. Impossible to continue without crushing your own self esteem and ignoring your own justified needs.

This. Age difference between them would bother me more than the messages themselves.

Take away his (poor) excuses, and see it for what it is. He was messaging a much younger colleague behind your back, paying her compliments and calling her beautiful fully showing his interest in her. They were 1:1 messages, not in a group chat.
She would have felt desired by him and he knows it
He gaslit you and played it down, essentially lying to you. When you told him how you felt, he refused to do anything about it and WANTED (and did) continue to do it.
I do class this as cheating, and it’s unforgivable in my opinion. I could not trust him and I could not continue the relationship.

It's the work culture? Is he like this with all his other colleagues? Are they all private messaging each other flirting and telling each other how attractive they are? It's the culture to flirt with much younger members of staff?

Someone who loves you doesn't lie and gaslight you. You had perfectly reasonable questions about their relationship and he suggested you get counselling.

He was obviously getting off on a much younger woman paying him attention. I wonder what the real cause of the souring of their relationship was.

This would give me the ick.

It doesn't matter what we think. It DOES matter what you feel. The trust has gone and contempt has crept in.

I agree with OP's feelings of heartbreak and loss of trust and with what everyone is saying about the lying and the gaslighting plus I would worry about what, deep down, is his real culture and view of women if he relates to women in his office in this way and thinks it is fine. It is horrible behaviour and abuse of power. I feel for you OP.