Husband broke our boundaries twice with his ex while I was pregnant. How do I rebuild trust?

I’m 26F and my husband is 34M. We’ve been together for several years, and I’m currently 36 weeks pregnant with our second son. We also have a 17-month-old together.
Very early in our relationship, we agreed on two boundaries:
We wouldn’t communicate with people we had a romantic history with.
We wouldn’t use porn because neither of us likes the unrealistic expectations it can create.
When I was about 32 weeks pregnant with our first child, I discovered he had been talking to his first love (I’ll call her Carol). I found out because her Snapchat came up as a recent contact when he was trying to send documents at a car dealership.
He told me she had been trying to contact him and had even reached out to his family to make sure he was okay. He said he only responded to tell her he was fine and to stop contacting him. I was hurt because it violated a boundary we’d agreed on. I asked him to block her, and he admitted he was wrong, apologized, promised it wouldn’t happen again, and I chose to forgive him.
Now, about a year and a half later, I’m 36 weeks pregnant with our second baby.
For the past month he had seemed emotionally distant. I had an opportunity to look through his phone, and although I wasn’t expecting to find anything, I did.
On Reddit, I found several recently viewed NSFW pages, which was upsetting because of our agreement. More painful were two comments he had left: one asking someone to send him explicit content, and another telling a woman, “You’re really cute 🥺.” That’s something he says to me often, using the exact same wording and emoji.
Then I checked his recently deleted text messages and recovered a deleted conversation with Carol. There were hundreds of messages over about two weeks.
In those messages, he acknowledged he was risking a lot by talking to her and repeatedly said he wanted to keep talking to her. He told her that life feels like survival mode and that talking to her reminded him of a happier time in his life. They reminisced about their past relationship, and she even said, “Maybe in another life we stayed together.”
For context, they dated around eight years ago, and after they broke up they still stayed in contact. Even after she got married, they continued talking, and at one point he had planned to move out of state because she asked him to. So there is a significant emotional history between them.
Reading those messages felt like an emotional affair to me, and I now wonder if that’s why he had become emotionally distant from me.
When I confronted him, he didn’t deny anything. He apologized, admitted he was wrong, said I didn’t deserve to be hurt, and accepted responsibility.
The problem is that this isn’t the first time. I already forgave him once after he promised it would never happen again.
I’ve also been cheated on in every relationship before my husband, so this has brought back a lot of old wounds. I genuinely believed he was the one person I could fully trust.
I don’t know how to move forward. I love my husband, especially with another baby due in a few weeks, but I also don’t know how to rebuild trust after the same boundary has been broken twice.
TL;DR: My husband broke a relationship boundary by secretly talking to his ex while I was pregnant—twice. The second time included emotionally intimate conversations, deleted messages, and other boundary violations. He has apologized and taken responsibility, but I’m struggling to trust him again. How would you move forward in this situation? Is rebuilding trust realistic after this?