Unsure of what to make of this bomb my bf just dropped onto me

I (22F) have been with my boyfriend (24M) for 3 years, and we were about to move in together and get engaged—but recently everything feels shaken.

To start, he has always had this “girl best friend” in his friend group. Their families are really close and all call each other “cousins,” so I always saw it as a family-type relationship. They grew up talking constantly, FaceTiming, Snapchatting, and hanging out—even 1-on-1. They were also close at family events, and I even tried to befriend her.

Now, 3 years later, he told me that back in 2020 (2 years before me), they had sex one time at ages 17/18 after having feelings, but then went back to being friends. They continued talking and hanging out like normal after that.

What bothers me is that I had no idea that whole time. And during the first year of our relationship (2023) they were still communicating—Snapchatting (even in front of me and sending pictures of me), messaging like they had been before me. & I only recently found out they were also calling each other, often when I wasn’t around (and it seems like she was the one initiating it). She even came to visit him once with the “cousin group” and they all stayed in his apartment, and he says he just didn’t know how to tell her no without making it awkward for the group. There was even a framed picture in his apartment with a collage of them all, with the biggest photo being a selfie of them together (I’m pretty sure it was gifted). And I had no idea the whole time.

He also says that their calls were just quick, harmless catch-ups, and that she was even talking to his friend at the time, but now it feels off. They even had old nicknames & were super close. He also admitted that if roles were reversed, he wouldn’t be okay with it. On top of that, he would get upset if my guy friends texted me because he “knew they liked me,” yet he was still talking to a girl he had previously had sex with. I don’t know what to make of that.

They don’t talk anymore now—it faded out after our first year and completely stopped once she got a boyfriend (early 2024).

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The bigger issue:

Before we officially dated, we talked online for about a month as he lives out of state. During that time, he was still talking to and sleeping with another girl, which I understand since we weren’t exclusive—but I had no idea & he made it seem like I was the only one. For example, he would say things like, “the only girl who would ever be in this apartment would be you and my mom.” While having her over the whole time.

After we met in November 2022, things got serious fast—he met my family, posted me (first girl he posted in 3 years), and told people he was falling in love (I saw the messages). Although we weren’t technically “dating,” he was doing all the things. He flew back home after a month, and then a week later, he actually flew back in for two days to surprise me for my birthday. It honestly felt like a fairytale.

But he recently admitted that the same day he flew in for my birthday—after sleeping over and posting me—he went home and hung out with another girl he had previously had sex with (one time, two years prior). They grabbed food, and when she made advances, he ended up pulling over and making out with her (with sexual touching) before they both stopped. Then he came back to me that same night with flowers, acted completely normal, slept over again, and posted me the next day. I had no idea—for three years.

I reached out to the girl, and she confirmed the story. She said they would regularly hang out during that time and just grab food, and that one time it went too far.

The issue is that I asked him multiple times over the years if anything happened early on, and he always denied it until recently. He also downplayed his past a lot—lying about his body count and other things—because he had a “player” past but tried to present himself as this innocent, perfect man. I understand that no 20-year-old wants to fully lay out their past to someone they really like, especially since I was a virgin at the time and more innocent, and he knew that—but it still feels misleading.

After a recent big argument, he finally told me everything—the family friend, the birthday situation, his past—and said he was ashamed and scared to lose me. He says he has now laid everything out after carrying guilt for years and just wants a fresh start. He also said that after that situation happened, he knew he never wanted to put himself in that position again, and that it made him realize that he couldn’t keep acting like that if he was serious about me.

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Now:

He says that once we became official, he has been completely loyal, and I do believe he has grown a lot since then. He was 20 at the time with a “player” mindset that unfortunately carried into the beginning of our story, but now he’s 24. He has matured greatly, treats me really well, has grown in his faith, puts in consistent effort, and is very serious about our future. My friends always tell me how jealous they are because their boyfriends don’t even treat them as well as he has consistently treated me for 3 years!!! I also haven’t had to worry about any girls, social media, porn, anything really.

He has also flown in multiple times just to talk things through with me and is even suggesting couples therapy to work through this. He wants to get engaged soon. We share the same culture, religion, and values, and I do see him as someone I could marry.

But what hurts most is that I didn’t have the full truth back then to make my own decisions. Even though things now are so perfect, I catch myself ruminating on these new confessions.

So now I feel stuck—do I work through this because of who he is now, or is the lying/down playing early on something I shouldn’t ignore?